bazlets: Replace native.git_repository with skylark rule
bazlets: Replace native.git_repository with skylark rule

The native.git_repository rule does not work properly when invoked
in a `git submodule foreach` command. Bazel will deprecate the
native rules anyway, so replace with skylark's git_repository.

See [1] for context.

[1] https://github.com/bazelbuild/bazel/issues/6358#issuecomment-429103745

Change-Id: Icbbc0a747b48132a75ca20edcc6b24972e16b014

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