shared/install: when creating symlinks, accept different but equivalent symlinks

shared/install: when creating symlinks, accept different but equivalent symlinks

We would only accept "identical" links, but having e.g. a symlink
/usr/lib/systemd/system/foo-alias.service → /usr/lib/systemd/system/foo.service
when we're trying to create /usr/lib/systemd/system/foo-alias.service →
./foo.service is OK. This fixes an issue found in ubuntuautopkg package
installation, where we'd fail when enabling systemd-resolved.service, because
the existing alias was absolute, and (with the recent patches) we were trying
to create a relative one.

A test is added.
(For .wants/.requires symlinks we were already doing OK. A test is also
added, to verify.)