go_proto_library: default "compiler" misinterprets importpath value with semicolon package name suffix

### What version of rules_go are you using?

`v0.33.0`

### What version of gazelle are you using?

`v0.24.0`

### What version of Bazel are you using?

`v5.2.0`

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

Yes.

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

Linux, AMD64

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

Using the default "compiler" for go_proto_library, not using a custom `go_proto_compiler` target for custom generation options. I realize that is another approach to take, but wondered if the below was an issue or not.

### What did you do?

Supplied a `path;package` format value to the `importpath` attribute of a `go_proto_library` target. For example, `google.golang.org/genproto/googleapis/cloud/secretmanager/v1;secretmanagerpb`

### What did you expect to see?

The generated code in bazel output with directory name `google.golang.org/genproto/googleapis/cloud/secretmanager/v1`.

### What did you see instead?

The generated code in bazel output with directory name `google.golang.org/genproto/googleapis/cloud/secretmanager/v1;secretmanagerpb`.

The goal was to see if we could override the importpath and package name with go_proto_library targets. The generated package name is properly updated, but the resulting directory structure of the generated code incorrectly includes the `;package` section of the value.

I'm not sure if using `path;package` syntax is actually allowed with `importpath`. If it isn't, perhaps lets throw an error? If it is, perhaps we should fix the generated directory structure to trim the `;package` suffix.

Start at the go_proto_library importpath handling and the default compiler, using the reported path;package example and the stated rules_go, Gazelle, and Bazel versions to reproduce the output directory. Determine whether the suffix is supported; done means either a clear validation error for unsupported syntax or generated code under the path without the package suffix.

Written by the indexing model from the issue text.