Speed up USES=cargo ports ops

MASTER_SITES aliases processing in the framework is very inefficient
and the CRATESIO indirection is not worth it. Normally ports only
have a handful of sites at most, but USES=cargo currently adds one
site for each crate by necessity. The inefficiency suddenly matters
a lot.

By consuming MASTER_SITE_CRATESIO directly we can sidestep this
issue without losing anything.

Before:
$ time make -C www/zola -V MASTER_SITES >/dev/null

4.21 real         4.14 user         0.03 sys

After:
$ time make -C www/zola -V MASTER_SITES >/dev/null

0.60 real         0.58 user         0.02 sys