Frontend files are not migrated with `vaadin:migrate-to-p3` if in subdirectories

[vaadin-miki]

Using maven goal to migrate beverage-buddy from v10 to v14 in Windows does not produce valid .js files for frontend.

Steps to reproduce:

• get beverage buddy, switch to v10 branch; it has html files in src/webapp/frontend/src/...
• update dependencies - change version to 14.1.25, add vaadin-maven-plugin
• install node, npm and bower
• run vaadin:migrate-to-p3
• observe some modulizer errors and warnings about missing dependencies
• build success

Result:

• the html files from src/webapp/frontend/**/*.html are copied without changes to frontend/**
• no Java files are changed

Expected:

• html files are properly transformed into .js files and Java annotations are updated

Used:

• Vaadin 14.1.25
• Maven 3 bundled into IntelliJ 2020.1
• Java 10.0.2
• node recommended by Vaadin 14.1.25
• git-bash
• Windows 10 Pro

Observation: When html files are moved from their subdirectories directly into src/webapp/frontend, they are properly transformed into .js files, although the annotations on Java files are not updated.

[vaadin-miki]

This issue has also been raised quite some time ago on the forum: https://vaadin.com/forum/thread/17797778/13-to-14-migration