Refactor TribitsAdjustPackageEnables.cmake to get ready to transition to new data-structures (#63)

[bartlettroscoe]

Description

This PR is a refactoring of the module TribitsAdjustPackageEnables.cmake to make it easier to understand and navigate. This makes the code more maintainable and makes it easier to refactor to use the new combined TPL and internal package data-structures to complete #63.

Instructions to Reviewers

• This PR is mostly just moving code around but it also involves some renaming of vars and the usage of IN LISTS in some foreach() loops (see the detailed commit log messages for more details).
• NOTE: Please do not comment on the usage of IN LISTS in the foreach() statements involving the list vars <Package>_[FORWARD_][LIB|TEST]_[OPTIONAL|REQUIRED]_DEP_[TPLS|PACKAGES because all of those loops are going to be replaced with the new vars <Package>_[LIB|TEST]_DEFINED_DEPENDENCIES and <Package>_[LIB|TEST]_DEP_REQUIRED_<depPkg> in the next PR.

[bartlettroscoe]

Hello @KyleFromKitware, please have a look at the changes in the new commit 1c324c3 (#532) and please update your review.

I am curious how you would suggest formatting multi-line if() statement clauses.