xls_dslx_generate_cpp_type_files does not actually support deps

[mikex-oss]

Describe the bug
If the target DSLX src file depend on types (not constants) from its deps, the generated C++ will be invalid.

You will get error: use of undeclared identifier '<type defined in dependency>'.

Expected behavior
There should be a mechanism to include generated headers from dependencies.

Additional context
Related: #1610 would expose this problem more heavily. To date, we've limited the impact of this bug because commonly its constants from DSLX dependencies that are referenced. These are just turned into literals in the generated C++.

[mikex-oss]

This now works well enough. A few remaining opens to consider for the future (mentioned in that commit):

• In the initial version, all xls_dslx_cpp_type_library dependencies are expected to use the same parent namespace as the current target for the transpiler to resolve imported type references to the C++ type. There might be cases where you want to be able to provide a custom mapping instead from header to namespace.
• C++ type pretty-printing (ToString and ToDslxString) do not properly include DSLX import subject / C++ namespace. This was complicated by the dispatching logic, since the To*String functions always recurse to the leaf-level.
• On an implementation note, @richmckeever suggested we ideally have a way to work on the resolved Types instead of messing with the AST nodes and ColonRefs.