Support annotation processors

[cgrushko]

For example, auto_value needs to run an annotation processor as part of a build.

The BUILD file that bazel-deps produces for auto-value is something like:

java_library(
    name = "auto_value",
    visibility = [
        "//visibility:public",
    ],
    exports = [
        "//external:jar/com/google/auto/value/auto_value",
    ],
)

In order to support annotation processing, one needs to change it to

java_library(
    name = "auto_value",
    exported_plugins = [
        ":auto_value_plugin",
    ],
    visibility = [
        "//visibility:public",
    ],
    exports = [
        "//external:jar/com/google/auto/value/auto_value",
    ],
)

java_plugin(
    name = "auto_value_plugin",
    processor_class = "com.google.auto.value.processor.AutoValueProcessor",
    deps = [
        "//external:jar/com/google/auto/value/auto_value",
    ],
)

Note the exported_plugins and processor_class attributes.

Proposal:
A dependencies entry in the yaml file will contain a field named processor_class. When it's present, bazel-deps will create a BUILD file similar to the example above.

[johnynek]

this proposal seems fine to me. Want to start a PR?

[cgrushko]

Sure. Can you give me code pointers? Where to start modifying?

[greggdonovan]

Thanks! We can close this now that #93 is merged.

[cgrushko]

Niiiiice :D