shared/install: rework alias check and add test

shared/install: rework alias check and add test

This mostly reuses existing checkers used by pid1, so handling of aliases
should be consistent. Hopefully, with the test it'll be clearer what it
happening.

Support for .wants/.requires "aliases" is restored. Those are still used in the
wild quite a bit, so we need to support them.

See https://github.com/systemd/systemd/pull/13119 for a discussion of aliases
with an instance that point to a different template: this is allowed.