Given an object that wishes to have a top level defaulter, traverse the
object looking for nested fields that have defaulters and generate a
single function for that type that invokes all defaulters. The function
will have the name `SetObjectDefaults_NAME`.

Types use `// +k8s:defaulter-gen=true` to indicate they wish a defaulter
generated. If a function already exists with the desired name
`SetObjectDefaults_NAME` then no generation will occur. At a package
level, authors can bulk select the types to generate by setting the
value of the comment to the name of a field - all objects with that
field name without `// +k8s:defaulter-gen=false` defined on the type
will get a defaulter.

Because the defaulting behavior from conversions happens recursively,
all defaulters are expected to be invoked. We call these defaulters
"non-covering" (other defaulters may be invoked beneath them). The
defaulters we generate, by comparison, are "covering" - no nested
defaulters should be invoked. To distinguish between these two types, we
introduce the `// +k8s:defaulter-gen=covers` comment on a defaulter
function which will instruct the generator that the function should
terminate recursion.

This sets the stage for future defaulter generation from comments by
subsuming our existing generators