RFE: Use distfiles mirror and fallback also for URLs with tags

[raimue@…]

If a URL in master_sites has a tag, the distfiles mirror and the fallback sites are not considered.

For example:

master_sites    http://foo.example.org/:tag1 http://bar.example.org/:tag2
distfiles       foo:tag1 bar:tag2

In this example, foo should be downloaded from foo.example.org, tag2 from bar.example.org. But the distfiles mirror and the fallback site are not added.

[jmr@…]

Implementation attached.

[jmr@…]

Committed in r37914.