incompatible_disable_legacy_proto_provider: Remove legacy `dep.proto.` access to the proto provider

[lberki]

We are migrating off of legacy providers that are addressed by name to providers addressed: https://bazel.build/designs/skylark/declared-providers.html

This bug is for migrating the provider containing information about .proto files. The following changes are required in Starlark sources:

• Change the way the provider is accessed from ctx.attr.dep.proto. to ctx.attr.dep[ProtoInfo]
• Change the way the presence of provider is checked from hasattr(dep, "proto") to ProtoInfo in dep
• Change the way a rule requires a provider from providers = [["​proto"]] to providers = [[ProtoInfo]]

[philwo]

This flag was not flipped in time for the Bazel 0.23.0 release and will thus be postponed to Bazel 0.24.0.