Filter resource file names to avoid including the same file multiple times. (#883)

* Filter resource file names to avoid including the same file multiple times.
Addresses issue #882.

* Use a Set instead of a Map to filter Verilog files.

* Use canonical paths for file name comparison and unify name generation.
Provide a common method for copying resources to a directory to ensure the same resource ends up with the same name if it's copied by multiple clients.

* Reduce confusion - another absolute -> canonical switch.
Use the canonical path on the verilator command line for the filter additional Verilog sources.