roots: move to be a submodule of project

`roots` needs to be able to access private fields of project.

src/build_loop.rs

@@ -4,9 +4,9 @@
 use crate::builder;
 use crate::notify;
 use crate::pathreduction::reduce_paths;
+use crate::project::roots;
+use crate::project::roots::Roots;
 use crate::project::Project;
-use crate::roots;
-use crate::roots::Roots;
 use crate::watch::Watch;
 use std::sync::mpsc::Sender;
 

src/lib.rs

@@ -43,7 +43,6 @@ pub mod ops;
 pub mod osstrlines;
 pub mod pathreduction;
 pub mod project;
-pub mod roots;
 pub mod socket;
 pub mod watch;
 

src/project.rs

@@ -1,5 +1,7 @@
 //! Wrap a nix file and manage corresponding state.
 
+pub mod roots;
+
 use cas::ContentAddressable;
 use std::os::unix::ffi::OsStrExt;
 use std::path::{Path, PathBuf};