More compatability between bzlmod and WORKSPACE (#15)

- Create a `defs` library
- Create an empty report file
- Be consistent about this repo presenting the same build interface as
the generated repo

### Changes are visible to end-users: no

### Test plan

N/A.

Author: arrdem

BUILD.bazel

@@ -1 +1 @@
-# The root BUILD.bazel should be empty to minimize user-exposed deps
+exports_files(["report.json", "defs.bzl"], visibility = ["//visibility:public"])

extension.bzl

@@ -135,7 +135,7 @@ TELEMETRY = 1
 
     repository_ctx.file(
         "BUILD.bazel",
-        """
+        """\
 exports_files(["report.json", "defs.bzl"], visibility = ["//visibility:public"])
 """
     )

report.json

@@ -0,0 +1 @@
+{}