Rename everything to MetaCPAN::Client:

Since MetaCPAN::API is moving to CPAN-API Github organization,
MetaCPAN::API is not a very descriptive term. Instead, we rename
it to MetaCPAN::Client.

Some tests still use MetaCPAN-API as a search. This is because it
is still available on CPAN, while MetaCPAN::Client hasn't been
released yet.