[prost] Fix handling of message names with acronyms (#2193)

For example, the message name `SomeAPI`. Internally protoc-gen-prost
[converts](https://github.com/neoeinstein/protoc-gen-prost/blob/1a6d3593622af18b75a4a79f545f8530cdaf444f/protoc-gen-tonic/src/util.rs#L14)
this to upper camel case, but rules_rust is not doing that when
populating the `--extern_path` args. The result is that when a message
with an acroynym in its name is included in another proto file, the
generated code has a type name with the wrong casing.

Without this fix, the added test fails to build like this:

```
error[E0412]: cannot find type `NameWithCAPS` in module `camel_case_proto::camel_case`
 --> bazel-out/k8-fastbuild/bin/proto/prost/private/tests/camel_case/another_proto.lib.rs:9:75
  |
9 |         pub inner: ::core::option::Option<::camel_case_proto::camel_case::NameWithCAPS>,
  |                                                                           ^^^^^^^^^^^^ help: a struct with a similar name exists: `NameWithCaps`
 --> bazel-out/k8-fastbuild/bin/proto/prost/private/tests/camel_case/camel_case_proto.lib.rs:7:5
  |
  = note: similarly named struct `NameWithCaps` defined here

error: aborting due to previous error
```

5 files changed, 63 insertions, 2 deletions

diff --git a/proto/prost/private/protoc_wrapper.rs b/proto/prost/private/protoc_wrapper.rs
index bf74ae63..384c41f7 100644
--- a/proto/prost/private/protoc_wrapper.rs
+++ b/proto/prost/private/protoc_wrapper.rs
@@ -9,7 +9,7 @@ use std::path::PathBuf;
 use std::process;
 use std::{env, fmt};
 
-use heck::ToSnakeCase;
+use heck::{ToSnakeCase, ToUpperCamelCase};
 use prost::Message;
 use prost_types::{
     DescriptorProto, EnumDescriptorProto, FileDescriptorProto, FileDescriptorSet,
@@ -351,7 +351,7 @@ fn message_type_to_extern_paths(
 
     extern_paths.insert(
         proto_path.join(message_type_name),
-        rust_path.join(message_type_name),
+        rust_path.join(&message_type_name.to_upper_camel_case()),
     );
 
     let name_lower = message_type_name.to_lowercase();
diff --git a/proto/prost/private/tests/camel_case/BUILD.bazel b/proto/prost/private/tests/camel_case/BUILD.bazel
new file mode 100644
index 00000000..1720ad28
--- /dev/null
+++ b/proto/prost/private/tests/camel_case/BUILD.bazel
@@ -0,0 +1,37 @@
+load("@rules_proto//proto:defs.bzl", "proto_library")
+load("//proto/prost:defs.bzl", "rust_prost_library")
+load("//rust:defs.bzl", "rust_test")
+
+package(default_visibility = ["//proto/prost/private/tests:__subpackages__"])
+
+proto_library(
+    name = "camel_case_proto",
+    srcs = [
+        "camel_case.proto",
+    ],
+    strip_import_prefix = "/proto/prost/private/tests/camel_case",
+)
+
+proto_library(
+    name = "another_proto",
+    srcs = [
+        "another.proto",
+    ],
+    deps = [
+        ":camel_case_proto",
+    ],
+)
+
+rust_prost_library(
+    name = "another_proto_rust",
+    proto = ":another_proto",
+)
+
+rust_test(
+    name = "camel_case_test",
+    srcs = ["camel_case_test.rs"],
+    edition = "2021",
+    deps = [
+        ":another_proto_rust",
+    ],
+)
diff --git a/proto/prost/private/tests/camel_case/another.proto b/proto/prost/private/tests/camel_case/another.proto
new file mode 100644
index 00000000..186cfcf8
--- /dev/null
+++ b/proto/prost/private/tests/camel_case/another.proto
@@ -0,0 +1,9 @@
+syntax = "proto3";
+
+package another;
+
+import "camel_case.proto";
+
+message Another {
+    camel_case.NameWithCAPS inner = 1;
+}
diff --git a/proto/prost/private/tests/camel_case/camel_case.proto b/proto/prost/private/tests/camel_case/camel_case.proto
new file mode 100644
index 00000000..e472ddf5
--- /dev/null
+++ b/proto/prost/private/tests/camel_case/camel_case.proto
@@ -0,0 +1,7 @@
+syntax = "proto3";
+
+package camel_case;
+
+message NameWithCAPS {
+    string name = 1;
+}
diff --git a/proto/prost/private/tests/camel_case/camel_case_test.rs b/proto/prost/private/tests/camel_case/camel_case_test.rs
new file mode 100644
index 00000000..cc555680
--- /dev/null
+++ b/proto/prost/private/tests/camel_case/camel_case_test.rs
@@ -0,0 +1,8 @@
+//! Tests that strange casings of message names are handled correctly.
+
+use another_proto::another::Another;
+
+#[test]
+fn test_nested_messages() {
+    let _a = Another::default();
+}