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.
