this fixes cross deps such as "CHOST=armhf abuild deps"
to work properly.

if makedepends is not defined the following default
will be used (as that's the definition cross-build
aware apkbuilds use):
  makedepends="$makedepends_build $makedepends_host"

So we don't waste time installing dependencies on builder if
the host dependencies are not satisfiable.

supporting these features with cross building is non-trivial
and they do are generally not useful features, so remove them.
as result the abuildrepo is also removed to write out paths.
all package generation paths now use $REPODEST/$repo/$CARCH
which allows easily writing packages to correct $subpkgarch
in future commits.

For proper cross-build support, the subpackage arch needs to
be known before invoking the split function. This implements
a way to do that. This also changes to write the actual subpkg
arch to .PKGINFO - apk index --rewrite-arch still overwrites
index to have machine arch for noarch packages.