locale-util: add new helper locale_is_installed()
    
    This new helper checks whether the specified locale is installed. It's
    distinct from locale_is_valid() which just superficially checks if a
    string looks like something that could be a valid locale.
    
    Heavily inspired by @jsynacek's #13964.
    
    Replaces: #13964
    (cherry picked from commit 23fa786ca67ed3a32930ff1a7b175ac823db187c)
    
    Related: #1755287
    
    patch_name: 0484-locale-util-add-new-helper-locale_is_installed.patch
    present_in_specfile: true
    location_in_specfile: 484
    squash_commits: true