rules_scala `scala_proto_library` should respect `strip_import

Proto targets compiled with stripped prefixes are empty srcjars.

Minimal build file

proto_library(
    name = "example_proto",
    srcs = ["example.proto"],
    strip_import_prefix = "/prefix",
)

scala_proto_library(
    name = "example_scala_proto",
    deps = [":example_proto"],
)

Source proto:

# /prefix/example.proto
syntax = "proto3";
package foo
message Foo {}

Output:

$ bazel build prefix:example_scala_proto                                                                                                                                                                                                                                                                                               Sunday August 11 10:32:28 AM
INFO: Analyzed target //prefix:example_scala_proto (1 packages loaded, 3 targets configured).
INFO: Found 1 target...
INFO: From ScalaProtoCompile prefix/tmp/example_scala_proto:
protoc-jar: protoc version: 3.8.0, detected platform: osx-x86_64 (mac os x/x86_64)
protoc-jar: embedded: bin/3.8.0/protoc-3.8.0-osx-x86_64.exe
protoc-jar: executing: [/var/folders/t9/2yf73c553136pl7hzsrk8msm0000gn/T/protocjar10477993719091343746/bin/protoc.exe, --plugin=protoc-gen-scala=/var/folders/t9/2yf73c553136pl7hzsrk8msm0000gn/T/protocbridge3494382799574754229, --scala_out=bazel-out/darwin-fastbuild/bin/prefix/tmp/example_scala_proto, bazel-out/darwin-fastbuild/bin/prefix/_virtual_imports/example_proto/example.proto]
bazel-out/darwin-fastbuild/bin/prefix/_virtual_imports/example_proto/example.proto:3:1: Expected ";".
Target //prefix:example_scala_proto up-to-date:
  bazel-bin/prefix/example_scala_proto.srcjar
INFO: Elapsed time: 0.264s, Critical Path: 0.13s
INFO: 2 processes: 1 darwin-sandbox, 1 worker.
INFO: Build completed successfully, 4 total actions

The generated file (bazel-bin/prefix/example_scala_proto.srcjar) is empty.