Define abstract env-vars in package_info() for editable and unix_path

[memsharded]

Comes from #12192 (comment)_

Some packages want to define env-vars with paths, and they often use self.package_folder for it, so this is broken for editable packages, for example, in which package_folder=None. It also doesn't work for other abstractions over paths as unix_paths, which depend on the consumer, not the package creator.

Let's investigate some mechanism that allow to define abstract env-vars for PATHs that are resolved better in the consumer side.

[franramirez688]

This issue is related to #12477

[SpaceIm]

It also doesn't work for other abstractions over paths as unix_paths, which depend on the consumer, not the package creator.

Did #12598 fix this specific issue?