bake: avoid early-exit for resolution failures

With changes made to allow lazy evaluation, we were early exiting if an
undefined name was detected, either for a variable or a function.

This had two key implications:

1. The error messages changed, and became significantly less
   informative.

   For example, we went from:

   > Unknown variable; There is no variable named "FO". Did you mean "FOO"?, and 1 other diagnostic(s)

   To

   > Invalid expression; undefined variable "FO"

2. Any issues in our function detection from funcCalls which cause JSON
   functions to be erroneously detected cause invalid functions to be
   resolved, which causes new name resolution errors.

To avoid the above problems, we can defer the error from an undefined
name until HCL evaluation - which produces the more informative errors,
and does not suffer from incorrectly detecting JSON functions.

Signed-off-by: Justin Chadwell <me@jedevc.com>
(cherry picked from commit dc8a2b0398)

bake/hcl_test.go

@@ -670,6 +670,24 @@ func TestJSONFunctions(t *testing.T) {
 	require.Equal(t, ptrstr("pre-<FOO-abc>"), c.Targets[0].Args["v1"])
 }
 
+func TestJSONInvalidFunctions(t *testing.T) {
+	dt := []byte(`{
+	"target": {
+		"app": {
+			"args": {
+				"v1": "myfunc(\"foo\")"
+			}
+		}
+	}}`)
+
+	c, err := ParseFile(dt, "docker-bake.json")
+	require.NoError(t, err)
+
+	require.Equal(t, 1, len(c.Targets))
+	require.Equal(t, c.Targets[0].Name, "app")
+	require.Equal(t, ptrstr(`myfunc("foo")`), c.Targets[0].Args["v1"])
+}
+
 func TestHCLFunctionInAttr(t *testing.T) {
 	dt := []byte(`
 	function "brace" {