Error scaffolding external module if associated type not a ref

[matt2e]

Steps to repro:

• Create a module that uses an external ref with an associated type that isn't a ref
  • eg: builtin.CatchRequest[[]string]

Expected result:

• module should build with the correct associated types

Actual result:

• Usually the code succeeds because most types can be happily parsed as if they are refs
  • eg: string gets parsed as Ref{Name:"String", Module:""}
• If the type can not be parsed as a ref, then compiling causes a panic
  • eg: builtin.CatchRequest[[]string] because []string can not be parsed as a ref

panic is here: https://github.com/TBD54566975/ftl/blob/main/go-runtime/compile/build.go#L405