Prune export libraries and package lists

This pull request cleans up the cluttered export variables.

In `TribitsWriteClientExportFiles.cmake`, great lengths are gone to capture the entire dependency tree of every single package. None of this is actually necessary: Much of the dependency handling can be done by CMake itself. There is much room for cleaning up the export functionality, and this pull request makes a start by creating leaner export files.

Fixes bug #32.