install: when a template unit is instantiated via a /usr symlink, consider it enabled

install: when a template unit is instantiated via a /usr symlink, consider it enabled

If a unit foobar@.service stored below /usr is instantiated via a
symlink foobar@quux.service also below /usr, then we should consider the
instance statically enabled, while the template itself should continue
to be considered enabled/disabled/static depending on its [Install]
section.

In order to implement this we'll now look for enablement symlinks in all
unit search paths, not just in the config and runtime dirs.

Fixes: #5136