build: add default priority to ABI-versioned packages

build: add default priority to ABI-versioned packages

If a package has an ABI version defined, set priority to 10. The enables
packages with an ABI version to be installed by their base name instead
of a name and an ABI version, e.g.:

libfoo3, where 3 is the ABI version can be installed by just libfoo.

This affects manual installation only, as the dependency resolution
takes care of ABI versions.

Refactor apk priority logic into a helper define.

Signed-off-by: George Sapkin <george@sapk.in>
Link: https://github.com/openwrt/openwrt/pull/21369
Signed-off-by: Robert Marko <robimarko@gmail.com>