Fix DLL naming in metadata on Windows

Meson does not prefix the shared library names with `lib` on Windows, so these
internal metadata fields should not include it. And the Pidgin executable
wrapper needs to load this DLL via the un-prefixed name as well (via
`LIBPIDGIN_DLL_NAMEW`).

Testing Done:
Compiled on Windows.

Reviewed at https://reviews.imfreedom.org/r/2772/