Fix false warning that Download::GitHub is not being used and should.

[mohawk2]

This will fix #431 and PerlAlien/Alien-Build-Plugin-Download-GitHub#29.

[zmughal]

Tested this against the Alien-xz alienfile and it works.

The has_requires seems reasonable as a new public API method given that other plugins use it. But we may want exact matching instead of substring or regex (which gives you exact anyway with anchors). Or do:

$pattern_qr = ref $pattern eq 'Regexp' ? $pattern : qr/\Q$pattern\E/

The reason I would feel more comfortable with exact matching is that, here in the Alien-Build core, we can be specific about which plugin we are looking for.