sudoers: support `@includedir`

[Jorge Aparicio]

relevant tests:

sudo-rs/test-framework/sudo-compliance-tests/src/sudoers/includedir.rs

Lines 7 to 16 in 141709d

 #[test] 
 fn absolute_path() -> Result<()> { 
     let env = Env("@includedir /etc/sudoers.d") 
         .file("/etc/sudoers.d/a", SUDOERS_ALL_ALL_NOPASSWD) 
         .build()?; 
  
     Command::new("sudo") 
         .arg("true") 
         .output(&env)? 
         .assert_success() 

and the 17 tests that come after it