xls_dslx_generate_cpp_type_files does not actually support deps

**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 ''`.

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

**Additional context**
Related: https://github.com/google/xls/issues/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++.