merger: handle ident load (#2014)

**What type of PR is this?**
Bug fix

**What package or component does this PR mostly affect?**
merger

**What does this PR do? Why is it needed?**
I found that used idents in assignment expression rhs position have
their load statements removed. This PR adds those idents to used symbols
collection in `FixLoads` fn.

merger/fix.go

@@ -88,6 +88,12 @@ func FixLoads(f *rule.File, knownLoads []rule.LoadInfo) {
 			} else {
 				return
 			}
+		} else if ae, ok := x.(*bzl.AssignExpr); ok {
+			if id, ok := ae.RHS.(*bzl.Ident); ok {
+				idents = append(idents, id)
+			} else {
+				return
+			}
 		} else {
 			return
 		}

merger/fix_test.go

@@ -235,6 +235,21 @@ foo_binary(
     name = "a",
     field = magic_struct.member,
 )
+`,
+		},
+		"assignment expr rhs": {
+			input: `foo_binary(
+    name = "a",
+    field = magic_macro,
+)
+`,
+			want: `load("@bar", "magic_macro")
+load("@foo", "foo_binary")
+
+foo_binary(
+    name = "a",
+    field = magic_macro,
+)
 `,
 		},
 	} {