Downloadable file plugins should be able to handle local sources within the project

Summary

I don't think it's uncommon/unreasonable to expect that users may want to use a local file (e.g. a tarball) which they have locally, within their project. The downloadable file source plugins accept a url to the file (see here) and if we want to use a local file, we can specify a file url.

However, if I want to distribute my project, with the file, the absolute path within the file url will likely cause problems. Therefore I propose that we allow the downloadable source plugin to accept a path node, which, like the local source plugin is the relative path to the file from the project's root.

We should take care to ensure that the user specifies either url or path, and not both.