Use mark_as_advanced() everywhere that we call find_path() and find_library()

[osrf-migration]

Original report (archived issue) by Michael Grey (Bitbucket: mxgrey, GitHub: mxgrey).

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For more context: https://osrf-migration.github.io/ignition-gh-pages/#!/ignitionrobotics/ign-cmake/pull-requests/68/properly-mark-internal-cmake-cache/diff#comment-62213951