Strip a leading file:// (or file:///) scheme from the incoming path.
Cocoon sends full URIs like file:///<home>/.land/extensions/...
through FileSystem.ReadFile/WriteFile/ReadDirectory; PathBuf from
such a string treats the scheme literally and every read 404s. Without
this the redhat.java activation (and any other extension that uses the
gRPC fs.readFile path for its own package.json) fails with “Resource not
found: file:///…”.