Avoid including commands/dbcommands.h in so many places

Avoid including commands/dbcommands.h in so many places

This has been done historically because of get_database_name (which
since commit cb98e6fb8fd4 belongs in lsyscache.c/h, so let's move it
there) and get_database_oid (which is in the right place, but whose
declaration should appear in pg_database.h rather than dbcommands.h).
Clean this up.

Also, xlogreader.h and stringinfo.h are no longer needed by dbcommands.h
since commit f1fd515b393a, so remove them.

Author: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/202508191031.5ipojyuaswzt@alvherre.pgsql