Fix dep-info files emitting paths relative to deps' roots

Sample `shoo.d` file prior to this change is below, note the `build.rs`
at the end, which was not from my package.

From booping the debugger, I found this was coming from
`compiler_builtins`.  This is not really their bug though: if a build.rs
asks for rerun-if-changed on some crate relative path, this will happen
in general. So I've fixed it in Cargo and added a test to prevent it
regressing.

```
target/riscv64imac-mu-shoo-elf/release/shoo: /home/jade/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/../../stdarch/crates/core_arch/src/core_arch_docs.md /home/jade/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/../../stdarch/crates/core_arch/src/macros.rs /home/jade/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/../../stdarch/crates/core_arch/src/mod.rs /home/jade/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/../../stdarch/crates/core_arch/src/simd.rs /home/jade/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/../../stdarch/crates/core_arch/src/simd_llvm.rs crates/build_bits/src/lib.rs shoo/src/main.rs shoo/src/task.rs shoo/src/vectors.s build.rs
```

This change fixes it so it's like:

```
target/riscv64imac-mu-shoo-elf/release/shoo: /home/jade/.cargo/registry/src/github.com-1ecc6299db9ec823/compiler_builtins-0.1.39/build.rs /home/jade/.cargo/registry/src/github.com-1ecc6299db9ec823/log-0.4.14/build.rs /home/jade/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/../../stdarch/crates/core_arch/src/core_arch_docs.md /home/jade/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/../../stdarch/crates/core_arch/src/macros.rs /home/jade/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/../../stdarch/crates/core_arch/src/mod.rs /home/jade/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/../../stdarch/crates/core_arch/src/simd.rs /home/jade/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/../../stdarch/crates/core_arch/src/simd_llvm.rs crates/build_bits/src/lib.rs shoo/src/main.rs shoo/src/task.rs shoo/src/vectors.s
```

Author: lf-

src/cargo/core/compiler/output_depinfo.rs

@@ -81,6 +81,9 @@ fn add_deps_for_unit(
     if let Some(metadata) = cx.find_build_script_metadata(unit) {
         if let Some(output) = cx.build_script_outputs.lock().unwrap().get(metadata) {
             for path in &output.rerun_if_changed {
+                // The paths we have saved from the unit are of arbitrary relativeness and may be
+                // relative to the crate root of the dependency.
+                let path = unit.pkg.root().join(path);
                 deps.insert(path.into());
             }
         }

tests/testsuite/build_script.rs

@@ -1,8 +1,9 @@
 //! Tests for build.rs scripts.
 
-use cargo_test_support::paths::CargoPathExt;
+use cargo_test_support::project_in_home;
 use cargo_test_support::registry::Package;
 use cargo_test_support::{basic_manifest, cross_compile, is_coarse_mtime, project};
+use cargo_test_support::{lines_match, paths::CargoPathExt};
 use cargo_test_support::{rustc_host, sleep_ms, slow_cpu_multiplier, symlink_supported};
 use cargo_util::paths::remove_dir_all;
 use std::env;
@@ -2602,6 +2603,98 @@ fn fresh_builds_possible_with_multiple_metadata_overrides() {
         .run();
 }
 
+#[cargo_test]
+fn generate_good_d_files() {
+    // this is here to stop regression on an issue where build.rs rerun-if-changed paths aren't
+    // made absolute properly, which in turn interacts poorly with the dep-info-basedir setting,
+    // and the dep-info files have other-crate-relative paths spat out in them
+    let dep = project_in_home("awoo")
+        .file(
+            "Cargo.toml",
+            r#"
+                [project]
+                name = "awoo"
+                version = "0.5.0"
+                build = "build.rs"
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .file(
+            "build.rs",
+            r#"
+                fn main() {
+                    println!("cargo:rerun-if-changed=build.rs");
+                    println!("cargo:rerun-if-changed=barkbarkbark");
+                }
+            "#,
+        )
+        .build();
+
+    let p = project_in_home("meow")
+        .file(
+            "Cargo.toml",
+            &format!(
+                r#"
+                [project]
+                name = "meow"
+                version = "0.5.0"
+                [dependencies]
+                awoo = {{ path = "{path}" }}
+            "#,
+                path = dep.root().to_str().unwrap(),
+            ),
+        )
+        .file("src/main.rs", "fn main() {}")
+        .build();
+
+    p.cargo("build -v").run();
+
+    let dot_d_path = p.bin("meow").with_extension("d");
+    println!("*meow at* {:?}", dot_d_path);
+    let dot_d = fs::read_to_string(&dot_d_path).unwrap();
+
+    println!("*.d file content*: {}", &dot_d);
+
+    assert!(
+        lines_match(
+            "[..]/target/debug/meow: [..]/awoo/barkbarkbark [..]/awoo/build.rs[..]",
+            &dot_d
+        ) || lines_match(
+            "[..]/target/debug/meow: [..]/awoo/build.rs [..]/awoo/barkbarkbark[..]",
+            &dot_d
+        )
+    );
+
+    // paths relative to dependency roots should not be allowed
+    assert!(!dot_d.split_whitespace().any(|v| v == "build.rs"));
+
+    p.change_file(
+        ".cargo/config.toml",
+        r#"
+        [build]
+        dep-info-basedir="."
+    "#,
+    );
+    p.cargo("build -v").run();
+
+    let dot_d = fs::read_to_string(&dot_d_path).unwrap();
+
+    println!("*.d file content with dep-info-basedir*: {}", &dot_d);
+
+    assert!(
+        lines_match(
+            "target/debug/meow: [..]/awoo/barkbarkbark [..]/awoo/build.rs[..]",
+            &dot_d
+        ) || lines_match(
+            "target/debug/meow: [..]/awoo/build.rs [..]/awoo/barkbarkbark[..]",
+            &dot_d
+        )
+    );
+
+    // paths relative to dependency roots should not be allowed
+    assert!(!dot_d.split_whitespace().any(|v| v == "build.rs"));
+}
+
 #[cargo_test]
 fn rebuild_only_on_explicit_paths() {
     let p = project()