`URL.path` returns forward slashes in the path on Windows (https://github.com/swiftlang/swift-foundation/issues/973) where we expect backslashes. Work around that by defining our own `filePath` property that is backed by `withUnsafeFileSystemRepresentation`, which produces backslashes.
rdar://137963660
`buildTarget/inverseSources` is not required to be implemented by BSP servers and we have almost all information needed for it in `BuildSystemManager`.
This also makes sure that `buildTarget/sources` and `buildTarget/inverseSources` actually match each other. Before this change, we had source files like `Package.swift` for which we returned a target from `buildTarget/inverseSources` but that weren’t part of that target’s sources retrieved using `buildTarget/sources`.
