Hide duplicate names from extension views

Hide duplicate names from extension views

If extensions of equal names were installed in different directories
in the path, the views pg_available_extensions and
pg_available_extension_versions would show all of them, even though
only the first one was actually reachable by CREATE EXTENSION.  To
fix, have those views skip extensions found later in the path if they
have names already found earlier.

Also add a bit of documentation that only the first extension in the
path can be used.

Reported-by: Pierrick <pierrick.chovelon@dalibo.com>
Discussion: https://www.postgresql.org/message-id/flat/8f5a0517-1cb8-4085-ae89-77e7454e27ba%40dalibo.com