Instead of repeating the parsing stage after reaching the null
terminator, just handle it inside the loop and terminate the loop
afterwards.

We always replace that current pos with a null terminator, so it doesn't
matter what follows it.

This avoids looking for '/foo.pc'.

PKG_CONFIG_PATH paths must take precedence over the default path.
Otherwise, we would be unable to override default .pc files.

And while I'm at it, simplify the whole code. It is pointless to
introduce another buffer and a lot of string mangling for one additional
path.