Fix use of absolute paths to Swift SDK metatadata

The generator incorrectly recorded absolute paths to Swift SDK metadata in bundle archive metadata, which is fragile and also can't be handled by SwiftPM.

Author: MaxDesiatov

Sources/SwiftSDKGenerator/Generator/SwiftSDKGenerator+Metadata.swift

@@ -113,12 +113,16 @@ extension SwiftSDKGenerator {
         ArtifactsArchiveMetadata(
           schemaVersion: "1.0",
           artifacts: artifacts.mapValues {
-            .init(
+            var relativePath = $0
+            let prefixRemoved = relativePath.removePrefix(pathsConfiguration.artifactBundlePath)
+            assert(prefixRemoved)
+
+            return .init(
               type: .swiftSDK,
               version: self.bundleVersion,
               variants: [
                 .init(
-                  path: $0.string,
+                  path: relativePath.string,
                   supportedTriples: hostTriples.map { $0.map(\.triple) }
                 )
               ]