This method loads a "values file" -- also known as a "tfvars file" -- and
returns the values found inside.
A values file is an HCL file (in either native or JSON syntax) whose
top-level body is treated as a set of arbitrary key/value pairs whose
values may not depend on any variables or functions.
We will load values files through a configs.Parser -- even though values
files are not strictly-speaking part of configuration -- because this
causes them to be registered in our source code cache so that we can
generate source code snippets if we need to report any diagnostics.
Martin Atkins