Include repo_mapping in output of pkg_with_runfiles tarball.

[gonzojive]

Based on the work in bazelbuild/bazel#16124.

Tarball content before change:

 -r-xr-xr-x       0/0       1839256 foo/program
 -rw-rw-r--       0/0             0 foo/program.runfiles/_main/unused.txt
 -rw-rw-r--       0/0            13 foo/program.runfiles/data1~/message.txt

after

 -r-xr-xr-x       0/0       1839256 foo/program
 -rw-rw-r--       0/0             0 foo/program.runfiles/_main/unused.txt
 -r-xr-xr-x       0/0            74 foo/program.runfiles/_repo_mapping
 -rw-rw-r--       0/0            13 foo/program.runfiles/data1~/message.txt