emerge: enable parallel-fetch during pkg_pretend (bug 710432)

Execute pkg_pretend phases in a coroutine while parallel-fetch
is running concurrently. When it's time to execute the pkg_pretend
phase for a remote binary package, use a Scheduler _get_prefetcher
method to get a running prefetcher if available, and otherwise
start a new fetcher.

Since pkg_pretend phases now run inside of the --keep-going retry
loop, --keep-going is now able to recover from pkg_pretend
failures, which fixes bug 404157.

Bug: https://bugs.gentoo.org/404157
Bug: https://bugs.gentoo.org/710432
Signed-off-by: Zac Medico <zmedico@gentoo.org>
1 file changed