Refactor visibility options with #[darling(flatten)]

This reduces code duplication and makes it impossible for the options structs to see an invalid visibility state.

derive_builder/tests/compile-fail/vis_conflict.stderr

@@ -1,28 +1,16 @@
-error: `public` and `private` cannot be used together
- --> tests/compile-fail/vis_conflict.rs:7:15
-  |
-7 |     #[builder(public, private)]
-  |               ^^^^^^
-
-error: `vis="..."` cannot be used with `public` or `private`
+error: `public` and `vis` cannot be used together
  --> tests/compile-fail/vis_conflict.rs:5:25
   |
 5 | #[builder(public, vis = "pub(crate)")]
   |                         ^^^^^^^^^^^^
 
 error: `public` and `private` cannot be used together
-  --> tests/compile-fail/vis_conflict.rs:12:57
+  --> tests/compile-fail/vis_conflict.rs:12:48
    |
 12 | #[builder(public, vis = "pub(crate)", build_fn(private, public))]
-   |                                                         ^^^^^^
-
-error: `vis="..."` cannot be used with `public` or `private`
-  --> tests/compile-fail/vis_conflict.rs:14:30
-   |
-14 |     #[builder(private, vis = "pub")]
-   |                              ^^^^^
+   |                                                ^^^^^^^
 
-error: `vis="..."` cannot be used with `public` or `private`
+error: `public` and `vis` cannot be used together
   --> tests/compile-fail/vis_conflict.rs:12:25
    |
 12 | #[builder(public, vis = "pub(crate)", build_fn(private, public))]

derive_builder_core/Cargo.toml

@@ -21,7 +21,7 @@ clippy = []
 lib_has_std = []
 
 [dependencies]
-darling = "0.20.6"
+darling = "0.20.7"
 proc-macro2 = "1.0.37"
 quote = "1.0.35"
 syn = { version = "2.0.15", features = ["full", "extra-traits"] }