Support non-fully TriBITS-compliant external packages (#63)

[bartlettroscoe]

Description

This PR implements changes to TriBITS that allow it to handle non-fully TriBITS-compliant external packages, like the non-TriBITS Kokkos native CMake build system (see trilinos/Trilinos#11545).

Instructions to reviewers:

• The only significant changes to production code in this PR are in the files tribits/core/package_arch/TribitsInternalPackageWriteConfigFile.cmake and tribits/core/package_arch/TribitsProcessEnabledTpls.cmake . The rest of the commits are either mostly tests or documentation.
• It is likely better to review this code one commit at a time.

Remaining Tasks:

• [x] Add native TriBITS test for the Kokkos/Trilinos use case
• [x] Verify the 'demo-external-kokkos' Kokkos+KokkosKernels+Trilinos builds and use cases ... See trilinos/Trilinos#11863

[bartlettroscoe]

Hello @KyleFromKitware, do you have time to review this PR tomorrow so I can get this merged over the weekend? If not, we can set up for a post-merge review. There is actually very little production code changed here. It is likely better to review this PR one commit at a time.

[bartlettroscoe]

Hello @KyleFromKitware, can you please do a post-merge review of this PR?

[bartlettroscoe]

No more need for review so we can move into "done".