Some refinements and fixes (#63, trilinos/Trilinos#11976)

[bartlettroscoe]

@KyleFromKitware

Description

This PR branch addresses a few issues related to #63 and some other things:

• Updated tests and address case of dirty partially removed package directory and robust handling for ignoring missing packages when building for reduced tarball (see trilinos/Trilinos#11976)

• Do some cleanup of code related to (source) packaging and reduced tarball creation. Also added documentation for directly setting CPACK_SOURCE_IGNORE_FILES

• Factored out code for doing configure timing

• Print out found <Package>_DIR and assert it is set for TriBITS-compliant external packages

Instructions for reviewers

• This PR branch contains moves of larger chunks of code and should not be reviewed as a single patch
• Please review this PR one commit at a time

[bartlettroscoe]

@KyleFromKitware, if you have a few minutes today, could you please do a quick review of this PR? This should not take long to review it commit-by-commit. See "Instructions for reviewers" above.