Support packaging types: jar, war, aar, bundle, and so on

[jin]

Maven assumes jar by default [1]. https://maven.apache.org/pom.html#Maven_Coordinates

I propose that a new packaging field to be added as a child of artifactId, like so:

com.android.support.espresso:
  espresso-core:
    packaging: aar
    lang: java
    version: "3.0.0"

If packaging is not specified, it will default to jar [1]. This will unblock AAR support (#80).

I have a WIP commit here with passing tests: https://github.com/jin/bazel-deps/commit/ca00e226df2c74d5b515ea45c611ad5abb0e3fde#diff-ba7bf4242651f01e4c2b3dd54e4c3bbbR38

[1] .. or None, and let Aether internals figure that out itself.

[johnynek]

this seems like a great change. Would love to have it!