[ES6 modules] Integrate ModuleTreeLinkerRegistry into ModulatorImpl back-plane

This CL plumbs ModulatorImpl::FetchTreeInternal() to its algorithm implementation
ModuleTreeLinker{,Registry}.

BUG=594639

Review-Url: https://codereview.chromium.org/2826903002
Cr-Commit-Position: refs/heads/master@{#465799}
2 files changed