[pymtl/tools] fix bug with multi-import/translation for Verilog
[pymtl/tools] fix bug with multi-import/translation for Verilog

This commit fixes a bug when a user imported two different Verilog
modules into PyMTL and then tried to translate the composition.
Multi-import works, but the translation would only include one of the
imported Verilog files.

cornell-brg/pymtl,cornell-brg/pymtl,cornell-brg/pymtl