Do not crash on relative file URLs in a dependency (#3603)

Currently, SwiftPM crashes when someone specifies the URL of a package dependency as a relative file URL. We should instead be able to just make it absolute based on the location of the package itself.

Author: neonichu

Sources/PackageLoading/PackageDescription4Loader.swift

@@ -345,7 +345,7 @@ 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))).pathString
+                return AbsolutePath(String(dependencyLocation.dropFirst(filePrefix.count)), relativeTo: AbsolutePath(packageLocation)).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

@@ -499,6 +499,37 @@ class PackageDescription4_2LoadingTests: PackageDescriptionLoadingTests {
         }
     }
 
+    func testURLContainsNotAbsolutePath() throws {
+        let stream = BufferedOutputByteStream()
+        stream <<< """
+        import PackageDescription
+        let package = Package(
+            name: "Trivial",
+            dependencies: [
+                .package(url: "file://../best", from: "1.0.0"),
+            ],
+            targets: [
+                .target(
+                    name: "foo",
+                    dependencies: []),
+            ]
+        )
+        """
+
+        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")
+            }
+        }
+    }
+
     func testCacheInvalidationOnEnv() throws {
         #if os(Linux)
         // rdar://79415639 (Test Case 'PackageDescription4_2LoadingTests.testCacheInvalidationOnEnv' failed)