SAW core import fails for modules with overlapping imports

[eddywestbrook]

SAW core cannot currently handle a "module diamond", i.e., importing two modules which both import the same module. As a simplified example, if you start a SAW core file with

import Prelude; import Cryptol;

you will get the error

internal: Duplicate name "Array" being inserted into module linked_list

The issue appears to be that insImport in Verifier.SAW.Module calls insResolvedName on all names that are in scope in a module when that module is imported, including the names that module imports from other modules, and insResolvedName gets mad when the same name is inserted twice.

I think the easiest fix would be to make insResolvedName idempotent, so that two ResolvedNames with the same textual name can be added to the same module as long as they are equal. Another possibility would be to change either the calls to insImport or insImport itself to filter out ResolvedNames that are already in a module.