xref optional dependencies

[tsloughter]

I realized a drawback to how xref now only adds the runtime applications to analyze.

For apps with optional dependencies it means there is no way to include them in the analysis (aside from using a .app.src.script file).

Adding this to track the issue but I'm hoping we can just wait for erlang/otp#2675 but if it gets rejected we'll want to add a xref_additional_apps or something similar.

[ferd]

yeah currently there's xref_extra_paths that can sorta do that.