Make use of codecov_cli

[Daverball]

I think we can both improve our feature set and get rid of some reliability issues by using the same functions codecov_cli uses internally.

We will need to consider API stability however. So we may need to tightly couple ourselves against specific version of the CLI.

A fool-proof way would be to use subprocess or CLIRunner to directly invoke the CLI, but I think it may remove some of the tightly scoped control of what we're sending to CodeCov based on our current test-run only, so I think we're better off using the internal functions and giving it the data it requires based on the cov plugin state.