scala_proto should require JavaInfo provider and not specific rules

[ittaiz]

subj.
drive-by: removed old conditional.
@laurentlb @dslomov I really wanted to use ProtoSourcesProvider and not "proto" like the docs say but as far as I can see ProtoSourcesProvider isn't defined as a symbol. Also see bazelbuild/bazel#3701 and bazelbuild/bazel#3527
Can anyone help us move to the correct pattern?

[ittaiz]

by the way, my motivation came from the need to depend on scala_import and so that is the test I added. I decided to implement the generic best practice solution of providers since allow_rules is specific and deprecated. https://docs.bazel.build/versions/master/skylark/lib/attr.html

[ittaiz]

@johnynek any thoughts? I'd like to merge this since this is blocking us

[johnynek]

Thanks @ittaiz

[ittaiz]

Thank you!
@laurentlb @dslomov if we can still get your help we'd appreciate it since we'd like to avoid the usage of the legacy providers

[laurentlb]

@c-parsons
@lberki

[lberki]

@iirina