[5.9] Use a custom scheme for local "Declared In" source file URLs (#513)

Sources/SwiftDocC/SourceRepository/SourceRepository.swift

@@ -92,12 +92,15 @@ public extension SourceRepository {
 
     /// Creates a source repository hosted by the device's filesystem.
     ///
-    /// Use this source repository to format `file://` links to files on the
+    /// Use this source repository to format `doc-source-file://` links to files on the
     /// device where documentation is being presented.
+    ///
+    /// This source repository uses a custom scheme to offer more control local source file navigation.
     static func localFilesystem() -> SourceRepository {
         SourceRepository(
             checkoutPath: "",
-            sourceServiceBaseURL: URL(fileURLWithPath: "/"),
+            // 2 slashes to specify an empty authority/host component and 1 slash to specify a base path at the root.
+            sourceServiceBaseURL: URL(string: "doc-source-file:///")!,
             formatLineNumber: { line in "L\(line)" }
         )
     }

Tests/SwiftDocCTests/DocumentationService/ConvertService/ConvertServiceTests.swift

@@ -188,7 +188,7 @@ class ConvertServiceTests: XCTestCase {
 
             XCTAssertEqual(
                 renderNode.metadata.remoteSource?.url.absoluteString,
-                "file:///private/tmp/test.swift#L2"
+                "doc-source-file:///private/tmp/test.swift#L2"
             )
         }
     }

Tests/SwiftDocCTests/SourceRepository/SourceRepositoryTests.swift

@@ -88,7 +88,7 @@ class SourceRepositoryTests: XCTestCase {
         XCTAssertEqual(
             SourceRepository.localFilesystem()
                 .format(sourceFileURL: URL(string: "file:///path/to/file")!, lineNumber: 5),
-            URL(string: "file:///path/to/file#L5")!
+            URL(string: "doc-source-file:///path/to/file#L5")!
         )
     }
 }