install: refactor find_symlinks() and don't search for symlinks recursively
    
    After all we are only interested in symlinks either in top-level config
    directory or in .wants and .requires sub-directories.
    
    As a bonus this should speed up ListUnitFiles() roughly 3-4x on systems
    with a lot of units that use drop-ins (e.g. SSH jump hosts with a lot of
    user session scopes).