Related: #1967918
Also discussed in Fedora bug #1905584
Some extensions require to build with libpq as well, but this might be
a problem for the case server uses a private libpq, because extension
would pull in another libpq.so into the same process.
By adding new sub-packages private-libs and private-devel we can make sure
only a single copy of libpq is used by server, including the extensions.
Add dependency to libpq-devel to server-devel pkg if external libpq is used
in order to let extensions not to care about what libpq the server uses.
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