Dedupe all deps of cc_libraries

Without an explicit deduping in this helper function, a dependency may
appear multiple times in the generated BUILD files. I noticed this while
converting some internal google tests.

This currently does the deduping for the non-configurable root value. I
ran into some issues deduping the configurable values, and I will
address it in a followup CL.

Test: go test ./bp2build

Change-Id: Ie7b588a9c922d72e4fabfb1414a7cc3dc402ff86