Vendor dependencies

Let's see how I like this workflow.

vendor/bitflags/tests/compile-pass/visibility/pub_in.rs

@@ -0,0 +1,19 @@
+mod a {
+    mod b {
+        use bitflags::bitflags;
+
+        bitflags! {
+            pub(in crate::a) struct Flags: u32 {
+                const FLAG_A = 0b00000001;
+            }
+        }
+    }
+
+    pub fn flags() -> u32 {
+        b::Flags::FLAG_A.bits()
+    }
+}
+
+fn main() {
+    assert_eq!(0b00000001, a::flags());
+}