Load sub module frontend files from the file system instead of from the jar in dev mode

Use case in https://github.com/vaadin/flow/issues/607#issuecomment-524527810.

The reason this doesn’t work is that any frontend resources from jars are extracted into the main project’s file system when the servlet context is deployed and then webpack picks up the files from there.

For this kind of setup to work, it would be necessary to somehow configure webpack to also look for frontend files directly from the file system for project A in addition to the /frontend directory of the main project and then probably also generate imports in generated-flow-imports.js accordingly.

[Legioth]

it should be possible to have the application server pick up the changes from dependent modules when those are recompiled

That would require setting up the project structure in a way that first builds an updated jar file for the sub module and then redeploys the new project with an updated jar on the classpath. Doing all that would probably be too slow for a practical development flow. That’s why I suggested that it would instead somehow be possible to configure the dev server to pick those files directly from the file system instead of packaging and redeploying for each change.

[heruan]

Worth noting there are also dependencies to be taken care of: in Bower mode we had WebJars to depend on, in NPM mode we have package.json files around the JARs to be taken into account to fetch correct dependencies.