It seems not unusual for Linux distributions to supply libpq as part of a separate package (whether via dpkg, which I think uses "ar" as the archiver, or RPM, which uses cpio).
Possibly this is already provided on your system via some means akin to those.
If, instead, you are keen on getting the source code for libpq in a separate tarball, I'd seriously question why that would be expected to be valuable. On most systems, these days, it doesn't take terribly much time or space (on our systems with lots of GBs) to build all of Postgres, so separating the source code to the library out seems like an effort with not much value.
