Evaluating local variables used to be directly written to the
PackerConfig while each variable was created.
This was somewhat of an issue with testing, as we have a bunch of tests
that relied on `PackerConfig.Variables` being set only when we actually
write something.
This is not really a concern for normal use, just for testing, but to
limit the number of changes to the tests in hcl2template, I opted to
change how variables' values are retained, so that evaluating a single
variable returns a Variable in addition to hcl.Diagnostics, so we can
reify the approach and only create the map of variables if there's
something evaluated.
Lucas Bajolet
8d6b8da996