FindMPI: Fix finding PkgConfg for fallback

In !6537 (merged) the FindPkgConfig module was included directly. This produces warnings like:

The package name passed to `find_package_handle_standard_args` (PkgConfig)
does not match the name of the calling package (MPI).

Use find_package(PkgConfig) instead, as other find modules do.

Fixes: #22823 (closed)
Backport: release