Error on relative file URLs

It turns out that relative file URLs actually do not exist, see [here](#3604 (comment)) for more discussion. Instead we will now error and guide users to using a local package reference.

Author: neonichu

Sources/PackageLoading/PackageDescription4Loader.swift

@@ -345,7 +345,11 @@ extension PackageDependencyDescription {
             } else if dependencyLocation.hasPrefix(filePrefix) {
                 // FIXME: SwiftPM can't handle file locations with file:// scheme so we need to
                 // strip that. We need to design a Location data structure for SwiftPM.
-                return AbsolutePath(String(dependencyLocation.dropFirst(filePrefix.count)), relativeTo: AbsolutePath(packageLocation)).pathString
+                let location = String(dependencyLocation.dropFirst(filePrefix.count))
+                if location.first != "/" {
+                    throw ManifestParseError.invalidManifestFormat("file:// URLs cannot be relative, did you mean to use `.package(path:)`?", diagnosticFile: nil)
+                }
+                return AbsolutePath(location).pathString
             } else if URL.scheme(dependencyLocation) == nil {
                 // If the dependency URL is not remote, try to "fix" it.
                 // If the URL has no scheme, we treat it as a path (either absolute or relative to the base URL).

Tests/PackageLoadingTests/PD4_2LoadingTests.swift

@@ -516,17 +516,12 @@ class PackageDescription4_2LoadingTests: PackageDescriptionLoadingTests {
         )
         """
 
-        try loadManifestThrowing(stream.bytes) { manifest in
-            if let dep = manifest.dependencies.first {
-                switch dep {
-                case .scm(let scm):
-                    XCTAssertEqual(scm.location, "/best")
-                default:
-                    XCTFail("dependency was expected to be remote")
-                }
-            } else {
-                XCTFail("manifest had no dependencies")
-            }
+        do {
+            try loadManifestThrowing(stream.bytes) { _ in }
+            XCTFail("Unexpected success")
+        } catch ManifestParseError.invalidManifestFormat(let message, let diagnosticFile) {
+            XCTAssertNil(diagnosticFile)
+            XCTAssertEqual(message, "file:// URLs cannot be relative, did you mean to use `.package(path:)`?")
         }
     }