Formerly, `--scope-dir` would include only the specified directory in
the search path for already installed packages, etc., which means that
it would only work right as a kind of installation scope that is a
step beyond "installation" on the "user"-to-"installation" spectrum.
The `'pkgs-search-dirs` confiugration entry, meanwhile, provides more
control over search ordering in installation scope. Make `--scope-dir`
work more consistently with that search-path configration.
This change also makes "instllation"-scope operations use the search
path more consistently, since some actions used to use the whole
search list while others pruned any prefix before the main
installation directory in the search list.
0e2ad7d596