In some cases, rather than walking through a full block of HCL in order
to lift the references to other components of a build, we only have the
expression to walk on.
From this expression, we can lift the variables as a collection of
hcl.Traversal, from which we then filter only those that interest us.
So, since this is a use-case, we split the function that lifts the
expressions from a hcl block and filter them based on the root type of
the traversal into two separate functions.
Lucas Bajolet
aee537914a