go_proto_library rule with protoc plugin in compilers generates action conflict error.

### What version of rules_go are you using?

`v0.27.0`

### What version of gazelle are you using?

`v0.23.0`

### What version of Bazel are you using?

```
bazelisk version
Bazelisk version: v1.7.5
Build label: 4.0.0
Build target: bazel-out/k8-opt/bin/src/main/java/com/google/devtools/build/lib/bazel/BazelServer_deploy.jar
Build time: Thu Jan 21 07:33:24 2021 (1611214404)
Build timestamp: 1611214404
Build timestamp as int: 1611214404
```

### Does this issue reproduce with the latest releases of all the above?

Yes

### What operating system and processor architecture are you using?

`Ubuntu 20.04.2 LTS (Focal Fossa)`, `x86_64`

### Any other potentially useful information about your toolchain?

### What did you do?

I am currently trying to use with this [protoc-gen-gotag](https://github.com/srikrsna/protoc-gen-gotag) `protoc` plugin using compilers in `go_proto_library` rule.

Proto and BUILD files look like this :

```
syntax = "proto3";

option go_package = "github.com/xxx/yyy/src/services/my_service";

package my_service;

import "github.com/srikrsna/protoc-gen-gotag/tagger/tagger.proto";

message my_message {
int64 ID = 1 [(tagger.tags) = "db:\"id\"" ];
}
```

```
proto_library(
name = "my_service_proto",
srcs = ["my_service.proto"],
visibility = ["//visibility:public"],
import_prefix = "github.com/xxx/yyy/src/services/my-service",
deps = ["@com_srikrsna_protoc_gen_gotag//tagger:tagger_proto"],
)

go_proto_library(
name = "my_service_go_proto",
compilers = [
"@io_bazel_rules_go//proto:go_grpc",
":protoc_gen_gotag", # keep
], importpath = "github.com/xxx/yyy/src/services/my_service",
proto = ":manager_proto",
visibility = ["//visibility:public"],
deps = [
"@com_srikrsna_protoc_gen_gotag//tagger:tagger_go_proto"],
)

go_proto_compiler(
name = "protoc_gen_gotag",
plugin = "@com_srikrsna_protoc_gen_gotag//:protoc-gen-gotag",
valid_archive = False,
visibility = ["//visibility:public"],
deps = PROTO_RUNTIME_DEPS + WELL_KNOWN_TYPE_RULES.values() + WELL_KNOWN_TYPES_APIV2,
)
```

### What did you expect to see?

Build works with no error and `.pb.go` file is generated with correct annotations.

### What did you see instead?

Build is returning conflicting action error :

```
ERROR: file 'src/services/my_service/manager_go_proto_/github.com/xxx/yyy/src/services/my_service/my_service.pb.go' is generated by these conflicting actions:
Label: //src/services/my_service:my_service_go_proto
RuleClass: go_proto_library rule
Configuration: 86d23d2b5aedbeecfc6e6954ea9486ec39e9e40bcbc4b17cec46a06eed198d11
Mnemonic: GoProtocGen
Action key: d32f0d8df3a01a8485635fa88f513c4736142202d008892b4ee852a42b34a0ca, eef33c3e38791364fb33937110c1a07a77cef7fa93b58502e674936d39bb586e
Progress message: Generating into bazel-out/k8-fastbuild/bin/src/services/my_service/my_service_go_proto_/github.com/xxx/yyy/src/services/my_service
PrimaryInput: File:[[]bazel-out/k8-fastbuild/bin]external/com_google_protobuf/descriptor_proto-descriptor-set.proto.bin
PrimaryOutput: File:[[]bazel-out/k8-fastbuild/bin]src/services/my_service/my_service_go_proto_/github.com/xxx/yyy/src/services/my_service/my_service.pb.go
Owner information: ConfiguredTargetKey{label=//src/services/my_service:my_service_go_proto, config=BuildConfigurationValue.Key[86d23d2b5aedbeecfc6e6954ea9486ec39e9e40bcbc4b17cec46a06eed198d11]}
MandatoryInputs: Attempted action contains artifacts not in previous action (first 5):
src/services/my_service/protoc_gen_gotag_reset_plugin_/protoc-gen-gotag
Previous action contains artifacts not in attempted action (first 5):
external/io_bazel_rules_go/proto/go_grpc_reset_plugin_/protoc-gen-go
Outputs: are equal
```

I am feeling that the error happen because the `gotag` plugin is trying to modify the same file outputed by `protoc-gen-go` ?
But that's actually what the plugin is supposed to do.

As far as I understand, `go_proto_library` is [calling the registered compilers one after the other](https://github.com/bazelbuild/rules_go/blob/707c634bf8f7ca5b57c2a923bec16fa62ced1fc7/proto/def.bzl#L104), resulting in one `protoc` call for each provider.
I would have expected the rule to do only one `protoc` execution, with all registered plugins (compilers) specified.

Am I missing something ? Thanks :pray:

Reproduce the conflict with the supplied proto and BUILD definitions, then inspect proto/def.bzl at the linked compiler-handling code and the reported conflicting actions. Determine how multiple compilers affect the generated .pb.go output; done means the example builds successfully and generates the expected annotated file without an action conflict.

Written by the indexing model from the issue text.