Handle binary package IUSE_IMPLICIT divergence (bug 640318)

Since profile IUSE_IMPLICIT settings may diverge from those
that a binary package was built with, consider members of
built USE settings to be members of IUSE_EFFECTIVE. Only do
this for EAPIs that support IUSE_EFFECTIVE, since built USE
settings for earlier EAPIs may contain a large number of
irrelevant flags.

Note that the binary package may be remote, so it's only
possible to rely on metadata that is available in the remote
Packages file, and the IUSE_IMPLICIT header in the Packages
file is vulnerable to mutation.

Bug: https://bugs.gentoo.org/640318
4 files changed