fix(#586): flagging a pkgver out of date no longer flags newer versions

[m1rm]

This pull request:

• adds a version filter when flagging a package out of date: alpm.vercmp(peer.full_version, self.full_version) <= 0. This is the core fix for Flagging an old version should not flag newer versions #586
• refactors the core logic of flagging into flag_peers() which makes the code a bit more DRY
• adds a test for flagging packages behavior

Relates to: #586

[m1rm]

I think this PR and the associated bug can be closed because manually flagging is disabled? #719 cc @jelly

[jelly]

Thanks, makes sense!