This PR adds two new rules: copy_file and
copy_xfile.
Both rules solve a common problem: to copy one
file to another location. The problem is routinely
solved using a genrule. That however requires
Bash, since genrules execute Bash commands.
Requiring Bash is a problem on Windows.
The new rules do not require Bash on Windows (only
on other platforms).
The only difference between the rules is that
copy_xfile creates an executable file while
copy_file doesn't.
See https://github.com/bazelbuild/bazel/issues/4319
