Fix cmake warnings, FindPASTIX/FindPTSCOTCH.

Antonio Sánchezrequested to merge
cantonios/eigen:cmake into master

We were getting a lot of warnings due to nested find_package calls within Find***.cmake files. The recommended approach is to use find_dependency in package configuration files. I made this change for all instances.

Case mismatches between Find<Package>.cmake and calling find_package(<PACKAGE>) also lead to warnings. Fixed for FindPASTIX.cmake and FindSCOTCH.cmake.

FindBLASEXT.cmake was broken due to calling find_package_handle_standard_args(BLAS ...). The package name must match, otherwise the find_package(BLASEXT) falsely thinks the package wasn't found. I changed to BLASEXT, but then also copied that value to BLAS_FOUND for compatibility.

FindPastix.cmake had a typo that incorrectly added PTSCOTCH when looking for the SCOTCH component.

FindPTSCOTCH incorrectly added ***-NOTFOUND to include/library lists, corrupting them. This led to cmake errors down-the-line.

Fixes #2288 (closed).

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