Adopt Go example

Go module with a greet package and a stdlib-only solo package.

bazel run @rules_dx//:dx -- init //examples/adopt-go/...
bazel run @rules_dx//:dx -- generate //examples/adopt-go/...
bazel build //examples/adopt-go/...
bazel test //examples/adopt-go/...

Both tests pass (greet_test, solo_test).