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.
Fixes#10715
`config.Merge` was not updated to support a number of new features. This
updates the codepath to merge various fields, including the `terraform`
block which was the issue in #10715.
The `Merge` API is called when an `_override` file is present to _merge_
configurations. Normally configurations are _appended_. Only an override
file triggers a _merge_.
I started working on a generic library to do this automatically awhile
back but never finished it. This might motivate me to do so. In the
interest of getting a fix out though, we'll continue the manual
approach.
