Add some prerequisites

src/symbols/mod.rs

@@ -1,12 +1,17 @@
 use std::borrow::Cow;
 use std::error::Error;
 use std::fmt::Display;
+use resources::Resource;
 
 pub trait Symbol: Display {
   fn target_reached(&self) -> Result<bool, Box<Error>>;
   fn execute(&self) -> Result<(), Box<Error>>;
+  fn get_prerequisites(&self) -> Vec<Box<Resource>> {
+    vec![]
+  }
 }
 
+pub mod dir;
 pub mod file;
 pub mod git;
 pub mod npm;