install: only consider names in Alias= as "enabling"

install: only consider names in Alias= as "enabling"

When a unit has a symlink that makes an alias in the filesystem,
but that name is not specified in [Install], it is confusing
is the unit is shown as "enabled". Look only for names specified
in Alias=.

Fixes #6338.

v2:
- Fix indentation.
- Fix checking for normal enablement, when the symlink name is the same as the
  unit name. This case wasn't handled properly in v1.

v3:
- Rework the patch to also handle templates properly:
  A template templ@.service with DefaultInstance=foo will be considered
  enabled only when templ@foo.service symlink is found. Symlinks with
  other instance names do not count, which matches the logic for aliases
  to normal units. Tests are updated.