Split pkgver into components

[matteodelabre]

Our current pkgver field contains the epoch number, upstream version, and package version joined together. This makes it a bit more difficult than it should be to refer to the upstream version in recipes, for example to construct the source URL (see for example the Toltec recipes for Oxide or rm2fb). This also requires implementing a version parser to break those three components apart.

By splitting this field into three separate fields (like Arch’s PKGBUILD does), we could make both of these things simpler.