Candidate for kubectl integration

4d66f9a Pin to api v0.8.5
243e7cc Pin to cmd/config v0.9.7
711b4ff Pin to kyaml v0.10.15

a8b851f Reinstate configmap/secret generator env field (kustomization struct compatibility with v2.0.1)
f3a958b Extract flags that enable alpha function features (no starlark)
60bd8d1 upgraded to yaml.v2 v2.4.0
e9c97a4 updated go version to 1.16
722b013 return error for duplicate keys rather than panicking