modularize: Add code to find kdesrc-build modules.

This is the lead-in to work which will eventually split out kdesrc-build
into component Perl modules and complete the modularization process.

This commit makes it possible for Perl to locate kdesrc-build modules
that are located in the XDG app-data directory (in case kdesrc-build is
installed), with a fallback to the kdesrc-build script directory (in
case kdesrc-build is run from a git checkout).

Although I am "modularizing" kdesrc-build, this is not an attempt to
split into CPAN-quality modules, rather we're just splitting into
separate files modules that are already conceptually separate
internally. At this point kdesrc-build will still be a single package
using as few dependencies as possible. However this will make it
possible to clone useful Perl modules if needed and will continue on the
path of de-cluttering the main script.