Add the Version and Providers fields to the module config.
Add ProviderConfig.Scope, which will be used to record the original
path of a ProviderConfig for interpolation.
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.
This loader uses the HCL2 parser and decoder to process a config file,
and then transforms the result into the same shape as would be produced
by the HCL config loader.
To avoid making changes to the existing config structures (which are
depended on across much of the codebase) we first decode into a set of
HCL2-tailored structs and then process them into the public-facing structs
that a loader is expected to return. This is a compromise to keep the
config package API broadly unchanged for now. Once we're ready to remove
the old HCL loader (which implies that we're ready to support HCL2
natively elsewhere in the codebase) we will be able to simplify this
quite considerably.
Due to some mismatches of abstraction between HCL/HIL and HCL2, some
shimming is required to get the required result.
