Add support for import_prefix to protobuf rule

[Yannic]

This change switches closure_proto_library to use the binary descriptors returned by proto_library instead of invoking protoc directly on the source files.

To reduce the code duplication for Starlark *_proto_library implementations, rules_proto is used to generate JavaScript files from proto files.

Fixes #390

cc @sgammon

[sgammon]

@Yannic you are my hero, thank you!! i'll give it a shot

[sgammon]

@Yannic, sorry for the delay. i finally had a chance to check this out, and i figured out where to override it (we use rules_closure in a number of places in the project tree).

it does change the behavior, but, unfortunately, i get a crash of rules_closure. here's what i get as output:

ERROR: /private/var/tmp/_bazel_sam/.../external/project_with_protos/sources/.../BUILD.bazel:13:1: in @io_bazel_rules_closure//closure/protobuf:closure_proto_library.bzl%closure_proto_aspect aspect on proto_library rule @project_with_protos//sources/:proto_target: 
Traceback (most recent call last):
	File "/private/var/tmp/_bazel_sam/.../external/project_with_protos/.../BUILD.bazel", line 13
		@io_bazel_rules_closure//closure/protobuf:closure_proto_library.bzl%closure_proto_aspect(...)
	File "/private/var/tmp/_bazel_sam/.../external/io_bazel_rules_closure/closure/protobuf/closure_proto_library.bzl", line 49, in _closure_proto_aspect_impl
		proto_common.lang_proto_aspect_impl(actions = ctx.actions, toolchain =...], <6 more arguments>)
	File "/private/var/tmp/_bazel_sam/.../external/build_bazel_rules_proto/proto/proto_common.bzl", line 141, in proto_common.lang_proto_aspect_impl
		actions.run(inputs = depset(plugin_files, tr...]), <5 more arguments>)
Index 0 out of bounds for length 0

Obviously, the key here is Index 0 out of bounds for length 0. We have our own fork to test with now, at bloombox/rules_closure, so that's where I'll do some testing and see if I can dig in.

it looks like the code where this occurs is here in rules_closure, and it's calling into rules_proto here.

[Yannic]

@sgammon Thanks for trying this out!

I've found some issues with rules_proto when external workspaces are involved while porting some more *_proto_library rules. This should be fixed in rules_proto@head, so let me just update this PR :).

I'm not sure if that'll also fix the bug you're seeing. From what I've seen, there isn't any array access near the location the crash happened, and the error message looks like the Exception Java throws for out-of-bounds array access. What version of Bazel are you using?

[laurentlb]

Index 0 out of bounds for length 0
I think I've seen the error before when the set of inputs is empty.

[Yannic]

Thanks! Creating a proto_library without srcs reproduced it.

[Yannic]

Blocked on bazelbuild/rules_proto#6

[Yannic]

Withdrawing this for now. I'll send a new PR when [1] is accepted and implemented.

[1] https://docs.google.com/document/d/1u95vlQ1lWeQNR4bUw5T4cMeHTGJla2_e1dHHx7v4Dvg/edit