Support merging and relocating ProGuard rules

[Goooler]

What feature do you want to see added?

Files under META-INF/proguard/**.pro could be merged and relocated like the service files. Need to add a new ResourceTransformer for them.

[Goooler]

Relocating rules like:

-keep class com.foo.Bar {**}
-keep class com.foo.**

is possible, but it's hard to relocate more complex rules. I'm closing this for now until someone requests again.

[Goooler]

We don't have to rearrange the rules, just merge them lines by lines.