move subpackages to internal

Should we move all the subpackages into an `internal` subdir? They are not intended to be included directly, and we would be able to clean up this ugliness:
https://github.com/cockroachdb/cockroach/blob/dd4e4cac885e5486ea26bcc63df42139a09fbfca/pkg/testutils/lint/lint_test.go#L1774

CC @dhartunian 