Modules/Find{BLAS,LAPACK}: allow choice of pkg-config package

The BLA_PREFER_PKGCONFIG switch is not that useful if you are not able
to specify the pkg-config package to use. This adds BLA_PKGCONFIG_BLAS
and BLA_PKGCONFIG_LAPACK to that effect, allowing user choice in
environments that install differing variants of the BLAS libraries
with distinct .pc file names.

This is part of work to get more standardized installations of the
BLAS libs with specific names, likely blas.pc and lapack.pc only
for Netlib reference code, or maybe blas-netlib.pc and lapack-netlib.pc,
in any case distinct from choices like openblas-openmp.pc.
56 jobs for !7370 with drhpc-findblas-pkgconfig in 20 minutes and 16 seconds (queued for 3 seconds)
merge request