eclass: add ShadowedEclassPhase for multiple-eclass defined phases

[thesamesam]

Detect the basic case of an ignored eclass phase where two eclasses are inherited, both defining the same phase, and the ebuild does not define a custom implementation of that phase at all. This means one of the exported phases from the eclasses is being ignored.

Ignore some eclasses with a blacklist where they are known to vary their API by EAPI or eclass variables, at least for now, because the eclass cache we have accessible here isn't keyed by EAPI or the context of the sourcing ebuild.

Bug: https://bugs.gentoo.org/516014 Bug: https://bugs.gentoo.org/795006 Closes: https://github.com/pkgcore/pkgcheck/issues/377 Signed-off-by: Sam James [email protected]