It seems that this somehow got lost in the commit/rebase shuffle and
wasn't caught by the tests that _did_ make it because they were all using
just one file.
As a result of this bug, locals would fail to work correctly in any
configuration with more than one .tf file.
Along with restoring the append/merge behavior, this also reworks some of
the tests to exercise the multi-file case as better insurance against
regressions of this sort in future.
This fixes#15969.
We stash the locals in the module state in a map that is ignored for JSON
serialization. We don't include locals in the persisted state because they
can be trivially recomputed and this allows us to assume that they will
pass through verbatim, without any normalization or other transforms
caused by the JSON serialization.
From a user standpoint a local is just a named alias for an expression,
so it's desirable that the result passes through here in as raw a form
as possible, so it behaves as closely as possible to simply using the
given expression directly.
