Fix apk versioning

[kpenfound]

Fixes #474

Following examples from https://wiki.gentoo.org/wiki/Version_specifier as well as https://wiki.alpinelinux.org/wiki/APKBUILD_Reference#pkgver

I used Info.Prerelease as the pkgver.suf as described above, since that maps most similarly to the semver prerelease field. It potentially has some overlap with Info.VersionMetadata though. For parity with these fields for other package types in nfpm, it would maybe make sense to append both Prerelease and VersionMetadata to the version, however we would have to ensure that only one of the two fields is used.

Maybe it's better to expect the user to put {letter} and {suf{#}} in Info.Version instead.

[codecov]

Codecov Report

Merging #514 (7ac92ae) into main (1e26cfc) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #514      +/-   ##
==========================================
- Coverage   66.12%   66.09%   -0.04%     
==========================================
  Files          16       16              
  Lines        1916     1914       -2     
==========================================
- Hits         1267     1265       -2     
  Misses        510      510              
  Partials      139      139              

Impacted Files	Coverage Δ
apk/apk.go	71.91% <100.00%> (-0.16%)	⬇️

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[caarlos0]

LGTM, thanks!