pmb/aportgen/musl.py: fix distfile collisions (aportgen channels: part 2)

Parts 2-5 do not depend on each other. Total parts: 5

Add the Alpine mirrordir (e.g. "edge", "v3.12") to the distfile, so musl-*.apk files of the same version and arch but from different mirrordirs do not collide.

Let "abuild checksum" download these apks and generate the checksums, instead of letting apk download them as side-effect of initializing foreign arch chroots. The latter did not work anymore, because we would copy the apk file with a glob that may matches the hash of either mirrordir. Essentially:

glob.glob(f"{args.work}/cache_apk_{arch}/{subpkgname}-{version}.*.apk")[0]

In the context of the on-device installer, I found that calculating this hash is not trivial, so let's just avoid it here as well.

While at it, order the imports of musl.py alphabetically.

• Related: #1917 (comment 365136681)
• Related: !1939 (merged)