pmb.parse._apkbuild: Extend APKBUILD parser to work for subpackages

Until now we have only been able to parse the pkgdesc from subpackages, without any support for variable replacement. But we have a quite nice variable replacer now, so it would be nice to use it for subpackages as well :)

The end result of this MR is:

• Variable replacement for subpackage properties
• Parse all relevant subpackage properties (pkgdesc, depends, provides)
• We no longer assume the name of the subpackage function for kernel subpackage selection (related !1857 (merged))
• Various other (minor) fixes
• There is actually less code than before (excluding test cases) :)

See the commit log for details. It's probably easier to review the commits separately instead of looking at the combined changes.