Allow for custom npm packages in imported protos

Here is a link to the line where it speaks to our issue.
https://github.com/protocolbuffers/protobuf-javascript/blob/6113588085fea6149c51a750bd21b5cf9334a07d/generator/js_generator.cc#L116-L119

We currently make use of a variety of google protos other than those in the google-protobuf npm package. To make use of these, we have to explicitly include them in the npm package that we publish containing our generated protobufs. It would be ideal if we could publish the generated google protobufs we use in a separate npm package and use options in the .proto files to specify the name of the package to which these are published, and for this package to be used in the generated code whenever one of these protobufs is implemeneted. Similar to how this is for the "google-protobuf" imports

Start with generator/js_generator.cc at lines 116-119, linked in the issue, and trace how imports for google-protobuf are emitted. Determine how .proto options should specify package names for other generated protobuf packages; done means generated code uses those configured package names for the corresponding imports.