require_relative should never be used with bazel because it reads files from the source tree instead of the sandbox runfiles tree

I've discovered that the use of `require_relative` vs `require` actually reads files from the source tree instead of the files in the bazel sandbox runfiles tree.

Repro: https://github.com/bazel-contrib/rules_ruby/pull/224/files

In this example, you can remove the `:spec_helper` library from the `:add` test target and it will still pass even though spec_helper.rb doesnt exist in any of the runfiles. This is because require_relative loads the file directly from the source tree (your local repo) and bypasses Bazel sandbox.

If you change `require_relative` to `require`, it will fail to find the spec_helper.rb file until you add back `:spec_helper` as a dependency.

require always searches files from $LOAD_PATH, and in bazel rules_ruby, the deps, srcs, and bundle repo gems are always on in the list of dirs in $LOAD_PATH.

I think it might be good to add a require_relative static analysis check in `rb_library` to fail any rb file that has this function used in rules_ruby to avoid this silent/unintentional behavior. The result is that your bazel targets may be unknowingly using files that are not declared as dependencies. Thoughts?