Add scryer_path/1 manifest term for custom package search paths

Allows projects to specify additional manifest files whose package
directories should be searched via the scryer_path/1 term.

Example in scryer-manifest.pl:
    scryer_path(["vendor/scryer-manifest.pl"]).

Bakage searches for packages in this order:
1. SCRYER_PATH environment variable (if set)
2. Paths from scryer_path/1 in manifest (backtracking)
3. Default {project_root}/scryer_libs

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: jjtolton

README.md

@@ -28,6 +28,14 @@ dependencies([
     % A path to a local package
     dependency("test_local", path("./local_package"))
 ]).
+% Optional. Custom search paths for packages.
+% Specify paths to other scryer-manifest.pl files whose package directories
+% should be searched. Bakage will automatically determine where each manifest's
+% packages are located.
+scryer_path([
+    "vendor/scryer-manifest.pl",
+    "libs/other-project/scryer-manifest.pl"
+]).
 
 ```
 

bakage.pl

@@ -430,6 +430,37 @@
         append([RootChars, "/scryer_libs"], ScryerPath)
     ).
 
+% Get the current project's manifest (if it exists)
+current_project_manifest(Manifest) :-
+    find_project_root(Root),
+    append([Root, "/scryer-manifest.pl"], ManifestPath),
+    file_exists(ManifestPath),
+    parse_manifest(ManifestPath, Manifest).
+
+% Extract scryer_path/1 terms from manifest (list of manifest file paths)
+manifest_scryer_paths(Manifest, ManifestPaths) :-
+    member(scryer_path(ManifestPaths), Manifest),
+    ManifestPaths = [_|_].
+
+% Given a manifest file path, derive where its packages are located
+manifest_path_to_scryer_libs(ManifestPathChars, ScryerLibsPath) :-
+    join_sep_split(ManifestPathChars, "/", Segments),
+    append(DirSegments, [_ManifestFile], Segments),
+    join_sep_split(DirChars, "/", DirSegments),
+    append([DirChars, "/scryer_libs"], ScryerLibsPath).
+
+% Generate scryer path candidates via backtracking
+scryer_path_candidate(Path) :-
+    getenv("SCRYER_PATH", Path).
+scryer_path_candidate(ScryerLibsPath) :-
+    current_project_manifest(Manifest),
+    manifest_scryer_paths(Manifest, ManifestPaths),
+    member(ManifestPath, ManifestPaths),
+    manifest_path_to_scryer_libs(ManifestPath, ScryerLibsPath).
+scryer_path_candidate(Path) :-
+    find_project_root(RootChars),
+    append([RootChars, "/scryer_libs"], Path).
+
 % the message sent to the user when a dependency is malformed
 user_message_malformed_dependency(D, Error):-
     current_output(Out),
@@ -550,9 +581,10 @@
 
 package_main_file(Package, PackageMainFile) :-
     atom_chars(Package, PackageChars),
-    scryer_path(ScryerPath),
+    scryer_path_candidate(ScryerPath),
     append([ScryerPath, "/packages/", PackageChars], PackagePath),
     append([PackagePath, "/", "scryer-manifest.pl"], ManifestPath),
+    file_exists(ManifestPath),
     parse_manifest(ManifestPath, Manifest),
     member(main_file(MainFile), Manifest),
     append([PackagePath, "/", MainFile], PackageMainFileChars),