Help webjars locator to find assets in /META-INF/resources/webjars

[dsyer]

The webjars locator is really nice, and is configured automatically in Spring Boot, but it doesn't work in a native image because there is no directory traversal on the classpath (same problem with @ComponentScan). Maybe we can fix this in a similar way to @ComponentScan - the paths it scans are well defined and easy to discover. If we scan them at build time we can probably use a substitution at runtime to pick the right location, instead of trying to scan and failing. (See also oracle/graal#1108.)

[sdeleuze]

We could maybe use a similar build time transformation than the one described in #374.

[sdeleuze]

See spring-projects/spring-framework#27619.