pmb/helpers/pmaports: fix finding subpackages
Fix finding subpackages in pmaports, if no binary repo exists (-mp "" makes pmbootstrap not use a binary packages mirror for pmOS packages). Discovered with failing "test_depends_SLOW_40s" in bpo.
$ pmbootstrap -v -mp "" repo_missing --built --arch x86_64
[18:40:51] Calculate packages that need to be built (all packages, x86_64)
[18:40:56] ERROR: Package 'musl-dev-aarch64': Could not find aport, and could not find this package in any APKINDEX!- https://gitlab.com/postmarketOS/build.postmarketos.org/-/jobs/464793783
- https://postmarketos.gitlab.io/-/build.postmarketos.org/-/jobs/464793783/artifacts/_temp/local_job_logs/7801333.txt
Trivial and important fix, merging as soon as CI passes.
Edited by Oliver Smith