Generalize npm dependency support

[clehene]

All generated code currently generates js import paths with similar structure with that of the imported .proto files. The one exception for protobuf well-known types - google-protobuf based ont he import prefix:

string GetRootPath(const string& from_filename, const string& to_filename) {
  if (HasPrefixString(to_filename, "google/protobuf")) {
    // Well-known types (.proto files in the google/protobuf directory) are
    // assumed to come from the 'google-protobuf' npm package.  We may want to
    // generalize this exception later by letting others put generated code in
    // their own npm packages.
    return "google-protobuf/";
  }

https://github.com/grpc/grpc-web/blob/master/javascript/net/grpc/web/grpc_generator.cc#L461

This will generate something like

import * as google_api_annotations_pb from './google/api/annotations_pb';
import * as google_protobuf_timestamp_pb from 'google-protobuf/google/protobuf/timestamp_pb';

Given that the annotations are not part of the well-known types, and instead part of the google api common protos (e.g. https://github.com/googleapis/api-common-protos). This can cause problems.

An import option would allow users to explicitly override the relative path and specify a npm package at .proto import would be the right approach to generalize this.

Of course, because grpc-web and protobuf-js are more or less independent, this needs to be taken care in protobuf js generator as well, from where it was likely copied in the first place: https://github.com/protocolbuffers/protobuf/blob/master/src/google/protobuf/compiler/js/js_generator.cc#L128

string GetRootPath(const std::string& from_filename,
                   const std::string& to_filename) {
  if (to_filename.find("google/protobuf") == 0) {
    // Well-known types (.proto files in the google/protobuf directory) are
    // assumed to come from the 'google-protobuf' npm package.  We may want to
    // generalize this exception later by letting others put generated code in
    // their own npm packages.
    return "google-protobuf/";
  }

[loeffel-io]

need this option 🙏 +1