Allow TPLs to be listed in LIB_[REQUIRED|OPTIONAL]_PACKAGES (#63)

[bartlettroscoe]

Description

See the commit log and updated documentation for details.

For example, this should fix the SEACAS case described in https://github.com/sandialabs/seacas/pull/377#discussion_r1178233155. I.e. this will eliminate the need for some of the logic in:

https://github.com/sandialabs/seacas/blob/master/packages/seacas/libraries/ioss/cmake/Dependencies.cmake

Instructions for reviewers

• Review the updated logic in the function tribits_is_pkg_defined()
• Review the updated documentation:
  • tribits_package_define_dependencies() (see changes here)
  • How to add a new TriBITS Package dependency (see changes here)
• CHANGLOG.md entry (see changes here)

[bartlettroscoe]

@gsjaardema, this should fix the issue you reported in https://github.com/sandialabs/seacas/pull/377/files/75807545278f32397bbf6e4d0a1119f165a8a3a0#r1182980716.

If you have a few minutes, would you mind doing a review of the update documentation link to above under Instructions for reviewers?

[bartlettroscoe]

@KyleFromKitware, if you have a few minutes, would you mind doing a review? See Instructions for reviewers above.