pmb.helpers.package.get: no crash if dep missing

Fix the function, so it does not crash anymore when the replace_subpkgnames argument is set and a dependency cannot be resolved. Instead, print a useful warning that shows which pmaport caused the error (that has always been a pain to figure out), and simply don't replace the potential subpkgname with the real pkgname, just use the dependency name as-is.

Resolve annoying crashes in bpo dependency resolving, like this one (caused by a few linux-* pmaports for bad downstream kernels that depend on python, not apparent at all from the message):

[09:08:15] Calculate packages that need to be built (all packages, aarch64) [09:08:26] ERROR: Package 'python': Could not find aport, and could not find this package in any APKINDEX!

Example crash: https://builds.sr.ht/~postmarketos/job/184022#task-master_aarch64