For a file path, return a URL to the file if it exists and does not
represent a form evaluated at the REPL.

Resolve a relative javadoc path to a URL and return as a map. Prefer javadoc
resources on the classpath; then use online javadoc content for core API
classes. If no source is available, return the relative path as is.

If it's a resource, return a tuple of the relative path and the full resource path.