Move find_aport() and find_aport_guess_main() from pmb/build/other.py
to the new file pmb/helpers/pmaports.py.
Finding aports is not only needed when building packages, hence it
makes sense to move it out of pmb.build. The pmb/helpers/pmaports.py
file will have more pmaports related functions in a follow up commit.
Find the main package by assuming it is a prefix of the subpkgname. We
do that, because in some APKBUILDs the subpkgname="" variable gets
filled with a shell loop and the APKBUILD parser in pmbootstrap can't
parse this right. (Intentionally, we don't want to implement a full
shell parser.)