apk/abuild: Better error message when $pkgname is in $provides
We have found out, that when putting the `pkgname` in the `provides` variable and building the package, `apk` will not install it and fail with:
ERROR: unsatisfiable constraints:
Huh? Error reporter did not find the broken constraints.
I suggest showing an an appropriate error message in apk or - even better - directly in abuild when trying to build such an APKBUILD.
(Downstream bug report: <https://github.com/postmarketOS/pmbootstrap/issues/237>)
(from redmine: issue id 7586, created on 2017-07-23, closed on 2018-06-26)