Fix cmake warnings, FindPASTIX/FindPTSCOTCH.

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`](https://cmake.org/cmake/help/latest/module/CMakeFindDependencyMacro.html)
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.

cmake/FindBLAS.cmake

@@ -147,6 +147,7 @@ mark_as_advanced(BLAS_VERBOSE)
 
 include(CheckFunctionExists)
 include(CheckFortranFunctionExists)
+include(CMakeFindDependencyMacro)
 
 set(_blas_ORIG_CMAKE_FIND_LIBRARY_SUFFIXES ${CMAKE_FIND_LIBRARY_SUFFIXES})
 
@@ -509,9 +510,9 @@ if (BLA_VENDOR MATCHES "Intel*" OR BLA_VENDOR STREQUAL "All")
 
     if (_LANGUAGES_ MATCHES C OR _LANGUAGES_ MATCHES CXX)
       if(BLAS_FIND_QUIETLY OR NOT BLAS_FIND_REQUIRED)
-	find_package(Threads)
+	find_dependency(Threads)
       else()
-	find_package(Threads REQUIRED)
+	find_dependency(Threads REQUIRED)
       endif()
 
       set(BLAS_SEARCH_LIBS "")