pmb.parse._apkbuild: Extend APKBUILD parser to work for subpackages (!1866)

At the moment we have a simple subpkgdesc() function that can only
parse "pkgdesc" from subpackages, without support for any variables.
But we have a quite nice variable parser now that can be extended
to work for subpackages.

Simply put this works by:
  - Finding the lines that belong to the subpackage function
  - Stripping indentation (tab)
  - Parsing relevant attributes similar to the apkbuild() function

The "subpackages" in the parsed APKBUILD are replaced by a dict
of subpkgname: {"pkgdesc": "...", "depends": "..."} which are
parsed from the subpackage function (if found).
This makes it possible to get the "depends" of a subpackage.

test/test_helpers_package.py

@@ -49,7 +49,7 @@ def test_helpers_package_get_pmaports_and_cache(args, monkeypatch):
                 "provides": ["testprovide"],
                 "options": [],
                 "checkdepends": [],
-                "subpackages": [],
+                "subpackages": {},
                 "makedepends": [],
                 "pkgver": "1.0",
                 "pkgrel": "1"}