Absent values are omitted by the old code we are emulating in HCL, so we
must do the same here in order to avoid breaking assumptions in the
helper/schema layer.
The value-conversion machinery is also needed in the main "terraform"
package to help us populate our HCL2 evaluation scope, so a subset of the
shim functions move here into a new package where they can be public.
Some of them remain private within the config package since they depend
on some other symbols in the config package, and they are not needed
by outside callers anyway.