Add support for Closes tags with custom Bugzilla resolution

[antecrescent]

This PR is part of a larger effort[1] to automate resolving bugs on Gentoo's Bugzilla with the correct resolutions. The command line syntax would be:

• <bug>:<resolution> for a given bgo bug with an explicit resolution
• <bug>: or <bug> for any bug without a resolution (implicitly FIXED for a bgo bug)

Apart from FIXED, I chose to support:

• OBSOLETE e.g. for when older versions of a package get dropped which implicitly fixes a bug
• PKGREMOVED to encourage its adoption for the removal of packages

If you think any resolutions are missing here, please tell me!

I wasn't sure how to handle malformed URL-like input with multiple colons like https://foo:bar:obsolete. Currently, it gets converted into a Closes: https://foo:bar (obsolete) tag, similar to how -T/--tag does it.

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[1]: Patch for infra's githooks: https://bugs.gentoo.org/934825

[antecrescent]

Thanks for the feedback! I've been busy all week and I'll be implementing it on Sunday. Sorry for the delay!