rationalize bazel module-root/bazel-mapping flags in ddc

[jacob314]

There is some overlap between these flags. Likely both are required as a single module could contain files from genfiles and regular directories but it is possible there is an elegant solution to eliminate bazel-mapping.

[jmesserly]

how does this interact with #27262 ?

[jmesserly]

I think this is obsolete (or covered by other module-root bugs)