Generalize npm dependency support for generated js code

Sister issue of https://github.com/grpc/grpc-web/issues/584 (copying content)

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:
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/";
  }

Equivalent code in grpc-web:
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';

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

Start by comparing javascript/net/grpc/web/grpc_generator.cc around the referenced import-path logic with protobuf's src/google/protobuf/compiler/js/js_generator.cc. Determine the import option and package-path behavior needed to generalize the current google-protobuf exception. Done means generated imports can explicitly resolve dependencies from user-specified npm packages while preserving relative imports where no override is provided.