During implementation of HCL in other applications, it became clear that
the overloading of the word "attribute" to mean both a key/value pair in
a body and an element within an object value creates confusion.
It's too late to change that in the HCL Go API now, but here we at least
update the diagnostic messages. The new convention is that a key/value
pair within a block is now called an "argument", while an element of an
object is still called an "attribute".
It is unfortunate that the Go-facing API still uses the word "attribute"
for both, but the user experience is the most important thing and in
practice many applications will treat block arguments as one way to set
the attributes of some object anyway, and in that case arguments can be
thought of as the subset of attributes of an object whose values come
from that object's associated block.
This also includes a few other minor terminology tweaks in the diagnostic
messages the reflect how our lexicon has evolved during development and
authoring of user-facing documentation.
To assist in testing code that depends on hcl.ExprList and
hcl.AbsTraversalForExpr we now implement the necessary interfaces on our
existing MockExprLiteral and MockExprVariable, as well as adding new
functions MockExprList and MockExprTraversal that more directly serve
those interfaces with full functionality.
This is a super-invasive update since the "zcl" package in particular
is referenced all over.
There are probably still a few zcl references hanging around in comments,
etc but this takes care of most of it.
The main "zcl" package requires a bit more care because of how many
callers it has and because of its two subpackages, so we'll take care
of that one separately.
