We must ensure that the terraform required_version is checked as early
as possible, so that new configuration constructs don't cause init to
fail without indicating the version is incompatible.
The loadConfig call before the earlyconfig parsing seems to be unneeded,
and we can delay that to de-tangle it from installing the modules which
may have their own constraints.
TODO: it seems that loadConfig should be able to handle returning the
version constraints in the same manner as loadSingleModule.
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