benchmarks: rename to internal/benchmarks

I've come to agree with Joe about the proper location for this.

Change-Id: Ia5adbd1cd18f8cf40f7c3cc6bf8c7833dac37f20
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/214041
Reviewed-by: Joe Tsai <joetsai@google.com>
diff --git a/benchmarks/bench_test.go b/internal/benchmarks/bench_test.go
similarity index 100%
rename from benchmarks/bench_test.go
rename to internal/benchmarks/bench_test.go
diff --git a/benchmarks/download_benchdata.bash b/internal/benchmarks/download_benchdata.bash
similarity index 100%
rename from benchmarks/download_benchdata.bash
rename to internal/benchmarks/download_benchdata.bash