Introduce trait PackageCache

So we can add `TestPackageCache` and use it in our tests when resolving package definitions

Author: DavisVaughan

Cargo.lock

@@ -29,4 +29,5 @@ url.workspace = true
 aether_parser.workspace = true
 assert_matches.workspace = true
 oak_index.workspace = true
-oak_library = { workspace = true, features = ["testing"] }
+oak_library = { workspace = true, features = ["testing"] }
+oak_sources = { workspace = true, features = ["testing"] }