Repackager: pluggable layouts

[patrikbeno]

Repackager: layouts refactoring. Layouts are now pluggable: specify fully qualified name of the class implementing the Layout interface, or use known legacy aliases (JAR, ZIP, DIR, ...)

This effectivelly obsoletes #2619 and #2620.

Thanks @philwebb for idea.

This is initial draft. Submitting early because of those obsoleted requests that are being reviewed.

[patrikbeno]

Dedicating new package for layouts because referencing nested classes in Gradle is tricky (you need to use dollar which must be escaped in Gradle, and also, this is difference between Maven and Gradle, hence confusing...)

[patrikbeno]

ok, so now this actually obsoletes #2620 (see daa966c)

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[wilkinsona]

Thanks for the PR and sorry for the huge delay. As with #7263, we're not keen on the layouts no longer being strongly typed. Using a String means that people will lose auto-completion in their IDE and we want to avoid that. Our current preference is to keep our existing layout support as-is, but to allow a whole new layout "engine" to be plugged in. We will, by convention, pass some configuration into the engine, allowing more control over its behaviour.