Set new package deps graph vars combining external and internal packages (#63)

[bartlettroscoe]

Description

This PR sets up the new package dependency vars described in the section Variables defining the package dependencies graph (except the list names <Package>_FORWRD_[LIB|TEST]_DEP_PACKAGES was changed to <Package>_FORWRD_[LIB|TEST]_DEFINED_DEPENDENCIES for consistency with the new vars and to contrast with the old vars). This is another increment working to complete #63.

See the individual commits for more details.

Instructions for Reviewers

• It may be easier to review this PR one commit at a time (note next item)
• NOTE: There was a change made in the handling of <Project>_ASSERT_DEFINED_DEPENDENCIES and <Project>_ASSERT_MISSING_PACKAGES part way through the set of commits. (Therefore, be sure to compare with the final version of the file before putting any time into commenting an individual git commit patch.)

[bartlettroscoe]

Hello @KyleFromKitware, can you please do a full review of this PR and select "Approve" if you approve (or suggest changes)? Otherwise, I will be working on the next PR based in this PR's branch so there is no huge rush to get this merged.

[bartlettroscoe]

@KyleFromKitware, can you please have a look at the remaining unresolved conversations above here and here and the approve the PR if you are satisfied?

[bartlettroscoe]

@KyleFromKitware, thanks for the detailed review and the feedback! I need two write up a CMake programming conventions guide to cover all of these issues and decide the best way to handle corner cases.