add functionality to run relevant metadata tests for "repo-less" ebuilds

[radhermit]

This could either mean ebuilds sitting in a dir somewhere outside a repo or an ebuild piped into stdin. Most relevant checks would have to be reworked a bit to not assume they're working with a real pkg. It would probably even be easier to have a separate module of entirely stripped down metadata checks that would only be enabled in this specific situation.

Also, as we'd be faking a pkg, we'd probably need a new reporter that would take this into account and not output irrelevant data, such as the fake pkg's category.