Move tradefed_binary.go into subdir

Having two packages in the same directory makes it harder for go tools
to understande the directory structure.

For example, in build/soong, "go test ./cc/..." fails with cyclic
package imports.

Bug: n/a
Test: go test ./cc/... (in build/soong)
Change-Id: I7a040bf30b1e001680dd43fed9747e9651372274