depgraph: fix bug #526160

This fixes _dep_check_composite_db to mask packages that aren't the
highest visible match, but only if an update is desirable. This causes
desirable updates to get pulled in for cases like bug #526160. The
included unit test simulates the virtual/pypy update that triggered
the bug.

X-Gentoo-Bug: 526160
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=526160