Add a new package, `codegen/importer`, that can generate definitions for
resource states in PCL or TS/Python/C#/Go. The pipeline is relatively
simple: given a list of resource states, generate a PCL program in
memory, bind it, and pass it to the language-specific code generator.
This builds upon the existing PCL IR, and can be used with the currently
supported code generators.
Related to #1635.
