USE_GITHUB: For the new support, fix DISTNAME to be more consistent.
When using GH_TAGNAME the DISTNAME would have GH_PROJECT and GH_ACCOUNT in
it. When not using GH_TAGNAME it would not have this. Now both cases
will add in the GH_PROJECT and GH_ACCOUNT.
Add special care to ensure that the DISTVERSION is not added in twice. If
a port does GH_TAGNAME=v${PORTVERSION} it will be added in twice though. For
that case DISTVERSIONPREFIX=v should be set and no GH_TAGNAME should be used.
empty() is used rather than (!defined || !${}) to support fmake.
The purpose of setting DISTNAME at all in these cases is to make it more clear
that the distfile is from *GITHUB* and to avoid collisions if a project were
to be renamed or moved. Without adding in GH_PROJECT and GH_ACCOUNT then there
are real risks that collisions on filenames would happen on renamed or moved
projects, which is fairly common. A GITHUB-generated file may not match
a custom-rolled or git-archive-rolled distfile.
PR: 199069
With hat: portmgr
Testing done: All USE_GITHUB ports without GH_COMMIT were checksum/fetch/extract/WRKSRC tested.