allow CUR classname in RAKUDOLIB, PERL6LIB and -I

-Ifoo stills results in a CompUnitRepo::Local::File(foo), but now one
has the opportunity to get a CUR::Local::Installation by doing:
-ICompUnitRepo::Local::Installation=foo. Also, CURs only take a single
path now, since we can group every kind of CUR by putting it into an
array-group in @*INC.