Draft: pmb.helpers.pmaports.get/find: deduplicate code

find() gets the pmaports directory of a package, get() parses the APKBUILD inside that directory.

Instead of having the extra code path in get() for not searching subpackages, which duplicated part of find(), add a subpackages parameter to get().

Besides the deduplication done now, this is in preparation for code that will extend get() to handle the extra-repos/systemd directory - without this patch, the changes would need to be duplicated in both functions.

Related: #2328 (closed)