helpers.pmaports.find: fix subpackages logic
(Not urgent anymore, while pmaports!5619 failed in CI, the gcc upgrade is building fine in bpo.)
Fix that pmbootstrap can't find the dependency "musl-dev-aarch64" of "gcc-aarch64" in pmaports.
If we can't find the package by the full name in pmaports (in this case, musl-dev-aarch64 is provided by musl-aarch64), then we make a guess of what the main pkgname could be (as optimization, e.g. subpkgname-dev -> subpkgname), and then verify that guess.
However for musl-dev-aarch64 the guess does not work as it does not follow the usual packaging scheme, and so we must iterate through all APKBUILDs in pmaports and check if any of these provide musl-dev-aarch64 in subpackages.
Without this patch, going through all APKBUILDs only happens if we have made any guess before (which is None for musl-dev-aarch64).