Support numerics in PAUSE IDs

Prior to this patch,

    cpan -i IBMTORDB2/DBD-DB2-1.84.tar.gz

will end up trying to get I/I/I/IB/IBTORDB2/... because of double normalization.

As of today there are 4 PAUSE IDs with numbers in it: IBMTORDB2, P5P, PERL4LIB, WIN32
Merge remote-tracking branch 'remotes/miyagawa/numeric-pause-ids'