There are two expressions in HCL2 that are used to iterate over
collections:
- Splat expressions, e.g. `foo.*.bar`, and
- For expressions, e.g. `[for v in foo: v.bar]`
In both of these cases, the parts of the expression that are not the
collection being iterated behave like callbacks, and must be treated as
such by the apply rewriter.
Some of the apply rewriter's assumptions were broken by the richer
expressions available in HCL2. These changes fix those broken
assumptions, in particular the assumption that only scope traversal
expressions are sources of eventual values.