mojo: Move web-handling code under a dedicated modules/web subdir.

I keep it out of modules/ksb to avoid having it be confused with the
normal kdesrc-build logic, especially since Mojolicious will search
under its configure home directory for some kinds of searches. But, it
needs to stay under modules/ somewhere since I already fix the Perl
search paths to look here.

Having a modules/web seems like a decent compromise.