Rework how Rust generated files are laid out (#6731)

* Refactored Rust Generated code into a module directory.

Each symbol will be generated into one file and then
imported into a module. This breaks the "out_dir"
pattern where some users would generate code in their
target/ directory. Also, these objects are best used
in their own module. It will be hard for users to share
their own module structure with flatbuffers namespaces.

There may be solutions to these drawbacks but that should
be discussed. I don't want to overengineer here.

* shadow error

* try fix .bat file

* fix .bat 2

* Restore accidentally deleted files

* Fixed some DONOTSUBMITs and made Rust outdir pattern use symlinks.

* fixed binary files

* git clang format

* make generated onefiles not public and fix .bat

* reduced diff with master in generate_code.sh

* fix shadowed variable

* add object api flags to .bat

* space

* Removed extern crate and extra &

* use statement

* more clippy lints

* format

* Undo extern crate -> use change, it actually matters to our tests

Co-authored-by: Casper Neo <cneo@google.com>

samples/sample_binary.rs

@@ -19,10 +19,9 @@ extern crate flatbuffers;
 
 // import the generated code
 #[allow(dead_code, unused_imports)]
-#[path = "./monster_generated.rs"]
 #[allow(clippy::approx_constant)]  // We use low precision PI as a default value.
-mod monster_generated;
-pub use monster_generated::my_game::sample::{Color, Equipment,
+mod rust_generated;
+pub use rust_generated::my_game::sample::{Color, Equipment,
                                              Monster, MonsterArgs,
                                              Vec3,
                                              Weapon, WeaponArgs};