The preserved path is exposed by a new `module-path-index-submodule'
function, and `module-path-index-join' now accepts an optional
submodule path.
Also, fixed a problem with `collapse-module-path-index' when
a module path indx is built on a resolved module path that
is a submodule path.
In addition to the main repair, `collapse-module-path[-index]' is
correctly documented to allow '(quote <sym>) rel-to paths.
Finally, `collapse-module-path-index' changed to use a symbolic
resolved module path that appears as the base of a module path
index, rather than falling back to the given rel-to path. It's
possble that the old beavior was intentional, but it wasn't tested,
and it seems more likely to have been a bug.
Closes PR 12724
