Polymer Template cannot be found when inside a dependecy

Description of the bug

Polymer Template cannot be found as soon as it is part of a dependency. The reason for this might be my own fault f.e. maybe @HTMLImport should not be used by addons?, or rather the lack of documentation for this very specific case. I would be really glad if somebody could help me out on this.

Minimal reproducible example

An example project can be found here to get the two behaviours:

• run an install goal on the parent module
• execute jetty:run on the addon module the template is found
• execute jetty:run on the demo module the template cannot be found

Expected behavior

The template should be found.

Actual behavior

java.lang.IllegalStateException: Can't find resource 'context://iron-dropdown-wrapper/iron-dropdown-animation.html' via the servlet context
	at com.vaadin.flow.component.polymertemplate.DefaultTemplateParser.getTemplateContent(DefaultTemplateParser.java:104)
	at com.vaadin.flow.component.polymertemplate.TemplateDataAnalyzer.parseTemplate(TemplateDataAnalyzer.java:185)
	at com.vaadin.flow.component.polymertemplate.TemplateInitializer.<init>(TemplateInitializer.java:93)
	at com.vaadin.flow.component.polymertemplate.PolymerTemplate.<init>(PolymerTemplate.java:97)
	at com.vaadin.flow.component.polymertemplate.PolymerTemplate.<init>(PolymerTemplate.java:112)
	at com.github.appreciated.dropdown.IronDropdownWrapper.<init>(IronDropdownWrapper.java:30)
	at com.github.appreciated.dropdown.IronDropdownWrapper.<init>(IronDropdownWrapper.java:27)
	at com.github.appreciated.MainView.getIronDropDown(MainView.java:54)
	at com.github.appreciated.MainView.<init>(MainView.java:30)
	... 48 more

[pleku]

But it isn’t found in the addon module itself anymore

I think you need to define the resourceBase for the add-on module, see: https://github.com/vaadin/vaadin-grid-flow/blob/master/pom.xml#L123

[pleku]

The tutorial https://vaadin.com/docs/v10/flow/web-components/creating-an-in-project-web-component.html describes how to make a component for a webapp project. There the files are put to the /webapp/ folder. I actually don’t see any place where we mention that where you should put the files when you make an add-on that has local resources that should go to a jar artifact.

Maybe we should add documentation / tutorial for this, if this was the issue. Thus an issue for vaadin/flow-and-components-documentation repository would be great.