Preconditions and postconditions for resources and data sources may not
refer to the address of the containing resource or data source. This
commit adds a parse-time validation for this rule.
This allows precondition and postcondition checks to be declared for
resources and output values as long as the preconditions_postconditions
experiment is enabled.
Terraform Core doesn't currently know anything about these features, so
as of this commit declaring them does nothing at all.
This construct of a block containing a condition and an error message will
be useful for other sorts of blocks defining expectations or contracts, so
we'll give it a more generic name in anticipation of it being used in
other situations.
Alisdair McDiarmid