Don't rely on deps property position

Dependency errors were prefixed with ??? because they were associated
with the position of the "deps" property, which is often not used.  Use
the position of the module instead.
diff --git a/context.go b/context.go
index af81674..adeea4a 100644
--- a/context.go
+++ b/context.go
@@ -1112,12 +1112,10 @@
 }
 
 func (c *Context) addDependency(module *moduleInfo, depName string) []error {
-	depsPos := module.propertyPos["deps"]
-
 	if depName == module.properties.Name {
 		return []error{&Error{
 			Err: fmt.Errorf("%q depends on itself", depName),
-			Pos: depsPos,
+			Pos: module.pos,
 		}}
 	}
 
@@ -1126,7 +1124,7 @@
 		return []error{&Error{
 			Err: fmt.Errorf("%q depends on undefined module %q",
 				module.properties.Name, depName),
-			Pos: depsPos,
+			Pos: module.pos,
 		}}
 	}
 
@@ -1145,7 +1143,7 @@
 		Err: fmt.Errorf("dependency %q of %q missing variant %q",
 			depInfo.modules[0].properties.Name, module.properties.Name,
 			c.prettyPrintVariant(module.dependencyVariant)),
-		Pos: depsPos,
+		Pos: module.pos,
 	}}
 }
 
@@ -1182,14 +1180,12 @@
 func (c *Context) addVariationDependency(module *moduleInfo, variations []Variation,
 	depName string, far bool) []error {
 
-	depsPos := module.propertyPos["deps"]
-
 	depInfo, ok := c.moduleGroups[depName]
 	if !ok {
 		return []error{&Error{
 			Err: fmt.Errorf("%q depends on undefined module %q",
 				module.properties.Name, depName),
-			Pos: depsPos,
+			Pos: module.pos,
 		}}
 	}
 
@@ -1217,7 +1213,7 @@
 			if module == m {
 				return []error{&Error{
 					Err: fmt.Errorf("%q depends on itself", depName),
-					Pos: depsPos,
+					Pos: module.pos,
 				}}
 			}
 			// AddVariationDependency allows adding a dependency on itself, but only if
@@ -1226,7 +1222,7 @@
 			if depInfo == module.group && beforeInModuleList(module, m, module.group.modules) {
 				return []error{&Error{
 					Err: fmt.Errorf("%q depends on later version of itself", depName),
-					Pos: depsPos,
+					Pos: module.pos,
 				}}
 			}
 			module.directDeps = append(module.directDeps, m)
@@ -1238,7 +1234,7 @@
 		Err: fmt.Errorf("dependency %q of %q missing variant %q",
 			depInfo.modules[0].properties.Name, module.properties.Name,
 			c.prettyPrintVariant(newVariant)),
-		Pos: depsPos,
+		Pos: module.pos,
 	}}
 }
 
@@ -1316,7 +1312,7 @@
 				Err: fmt.Errorf("    %q depends on %q",
 					curModule.properties.Name,
 					nextModule.properties.Name),
-				Pos: curModule.propertyPos["deps"],
+				Pos: curModule.pos,
 			})
 			curModule = nextModule
 		}