js/{closure,commonjs}: create a new js_module option

What language does this apply to?

If it's a proto syntax change, is it for proto2 or proto3? proto3
If it's about generated code change, what programming language? javascript/commonjs but it can be applied to es6 or closure as well.

Describe the problem you are trying to solve.

We are generating code of multiple packages that we want to publish independently but with references between them. Currently the generator creates a tightly coupled relative paths between folders.

Describe the solution you'd like

An option like Go's go_package or C#'s csharp_namespace where I can put the import path, in this case the JS module.

I think a js_module called option would do the trick.

Describe alternatives you've considered

Overriding the generated code.

Start by reading the JavaScript generator's handling of relative imports for CommonJS, ES6, and Closure output, then review the discussion on this issue for the proposed js_module semantics. Done means an option can set independent module import paths while preserving references between generated packages across the supported JavaScript targets.