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Disable automatic skip-satisfied for arguments with version range.

Carton always pass --skip-satisfied when necessary, and doesn't need
automatic enabling of this option.
Retire search.cpan.org search!
Update the doc to reflect the change of not using s.c.o anymore
Explicitly pass skip-satisfied
Enable metacpan support by default.

MetaCPAN is by default queried after cpanmetadb since cpanmetadb can
serve requests much faster without doing a complicated index lookup,
but is preferred when given a version requirements, since only
MetaCPAN can serve such requests.

--metacpan option is undocumented but keeps captuting the option to
make it backward compatible.