interp: fix type check on function signature

Perform function declaration type check from the upper level scope (the scope where the
function is declared), to avoid possible collisions of local variables with package names.

Fixes #957.

_test/time14.go

@@ -0,0 +1,20 @@
+package main
+
+import (
+	"fmt"
+	"time"
+)
+
+var t time.Time
+
+func f() time.Time {
+	time := t
+	return time
+}
+
+func main() {
+	fmt.Println(f())
+}
+
+// Output:
+// 0001-01-01 00:00:00 +0000 UTC

interp/cfg.go

@@ -1381,7 +1381,7 @@ func (interp *Interpreter) cfg(root *node, importPath string) ([]*node, error) {
 			n.val = sc.def
 			for i, c := range n.child {
 				var typ *itype
-				typ, err = nodeType(interp, sc, returnSig.child[1].fieldType(i))
+				typ, err = nodeType(interp, sc.upperLevel(), returnSig.child[1].fieldType(i))
 				if err != nil {
 					return
 				}

interp/scope.go

@@ -113,6 +113,14 @@ func (s *scope) pop() *scope {
 	return s.anc
 }
 
+func (s *scope) upperLevel() *scope {
+	level := s.level
+	for s != nil && s.level == level {
+		s = s.anc
+	}
+	return s
+}
+
 // lookup searches for a symbol in the current scope, and upper ones if not found
 // it returns the symbol, the number of indirections level from the current scope
 // and status (false if no result).