Process CallbackDefineProjectPackaging.cmake before CallbackDefineRepositoryPackaging.cmake

Here I moved the processing of the repos's
CallbackDefineRepositoryPackaging.cmake files near the very end right before
INCLUDE(CPack) is called.  This is so that the code in the
CallbackDefineRepositoryPackaging.cmake files has access to all of the
project's setting needed to copy files on the source distribution tree.

The driver for this is that MPACT needs to include a generated file in the
tarball but to do that, the project-level CPACK vars like
CPACK_SOURCE_GENERATOR and CPACK_PACKAGE_NAME must be fully defined.

Note that I did not change any tests as part of this change.  That is because
the TribitsExampleProject file CallbackDefineRepositoryPackaging.cmake does
not need these vars and no other part of the testing checks the order of the
processing of these files.

After this commit, I will look at adding code to
CallbackDefineRepositoryPackaging.cmake that requires these vars to be set and
then will copy a file into the tarball and verify that it comes back out of
the tarball as well.

Build/Test Cases Summary
Enabled Packages: TriBITS
Enabled all Forward Packages
0) MPI_DEBUG => passed: passed=177,notpassed=0 (0.80 min)
1) SERIAL_RELEASE => Test case SERIAL_RELEASE was not run! => Does not affect push readiness! (-1.00 min)
Removing .egdist exclude for tarball

This file is not specific to core TriBITS and has no business being in default
exclude list.
Exclude non-processed extra repos from tarball

I don't have a test for this but there is not much to this.  I would need to
add a test of creating a tarball for a meta project and then asserting that
does the right thing.
Don't look for diff in .gitigore when missing from orig source tree (PHI Kanban #3529)

When TriBITS itself is copied into the tarball, TriBITS has default ignores
for .gitignore files so the test that looks for:

  Only in .*/TribitsExampleProject: .gitignore

fails when it is run from the untarred source.

This was easy to fix by only looking for this diff when .gitignore exists in
the source tree.

I verified that this test when run from the orgininal git source tree looks
for this diff output string in the tarball testing.
Update generated files for dev guide
Merge remote branch 'casl-dev/master'