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: Icd5a41d45155fb5607ae488cee58d6e654c490c5

GerritCodeReview/plugins_gitiles