hcl2: Use the actual string snippet as it appears from undefined

With the updated undefined variable code, we attempt to pick the text of
`${....}` verbatim from the hcl body. Previously, we'd attempt to
regenerate the string from the AST and pray it matches input; the
generation is lossy, as the same AST can represent multiple variations
(e.g. `${v.0}` and `${v[0]}` have the same HCLv2 AST). In this change,
we attempt to go back to the hcl2 source and find the string snippet
corresponding to the variable reference.

jobspec2/parse_test.go

@@ -855,6 +855,7 @@ func TestParse_UndefinedVariables(t *testing.T) {
 		"foo-${attr.network.dev-us-east1-relay-vpc.external-ip.0}",
 		`${env["BLAH"]}`,
 		`${mixed-indexing.0[3]["FOO"].5}`,
+		`with spaces ${   root.  field[  "FOO"].5  }`,
 	}
 
 	for _, c := range cases {

jobspec2/types.config.go

@@ -1,6 +1,7 @@
 package jobspec2
 
 import (
+	"bytes"
 	"fmt"
 	"strings"
 
@@ -308,8 +309,26 @@ func (c *jobConfig) EvalContext() *hcl.EvalContext {
 			inputVariablesAccessor: cty.ObjectVal(vars),
 			localsAccessor:         cty.ObjectVal(locals),
 		},
-		UnknownVariable: func(expr string) (cty.Value, error) {
-			v := "${" + expr + "}"
+		UndefinedVariable: func(t hcl.Traversal) (cty.Value, hcl.Diagnostics) {
+			body := c.ParseConfig.Body
+			start := t.SourceRange().Start.Byte
+			end := t.SourceRange().End.Byte
+
+			v := string(body[start:end])
+			if i := bytes.IndexByte(body[end:], '}'); i != -1 {
+				v += string(body[end : end+i+1])
+			} else {
+				// fallback for unexpected cases
+				v += "}"
+			}
+
+			if i := bytes.LastIndex(body[:start], []byte("${")); i != 0 {
+				v = string(body[i:start]) + v
+			} else {
+				// fallback for unexpected cases
+				v = "${" + v
+			}
+
 			return cty.StringVal(v), nil
 		},
 	}