Previously we were just loading the module and asserting no diagnostics,
but that is not really good enough since if we install modules incorrectly
it's possible that we are still able to load an empty configuration
successfully.
Now we'll do some basic inspecetion of the module tree that results from
loading what we installed, to ensure that all of the expected modules
are present at the right locations in the tree.
Unlike the old installer in config/module, this uses new-style
installation directories that include the static module path so that paths
we show in diagnostics will be more meaningful to the user.
As before, we retrieve the entire "package" associated with the given
source string, rather than any given subdirectory directly, because the
retrieved module may contain ../ references into parent directories which
must be resolvable after extraction.
Martin Atkins