Resolve dead_code and unused_variables warnings

Summary:
Rust 1.79's dead code scanner is more precise than previous versions. `pub` is no longer sufficient to hide a data structure field from the lint. It actually looks at whether an unused pub field is reachable from outside the crate.

In the following example, the field `field` is considered dead code by Rust 1.79 and not by 1.78.

```lang=rust
mod module {
    pub struct Struct {
        pub field: i32,
    }

    impl Struct {
        pub fn new() -> Self {
            Struct { field: 0 }
        }
    }
}

pub fn repro() {
    let _ = Struct::new();
}
```

Reviewed By: zertosh, JakobDegen

Differential Revision: D59623034

fbshipit-source-id: 6a4e2fb6e5be410d5127b451704df74ecdd42bf0

src/lockfile.rs

@@ -17,6 +17,7 @@ use crate::Paths;
 
 #[derive(Deserialize, Debug)]
 pub struct Lockfile {
+    #[allow(dead_code)]
     pub version: Hopefully3,
     #[serde(rename = "package")]
     pub packages: Vec<LockfilePackage>,

src/srcfiles.rs

@@ -33,19 +33,25 @@ pub struct Error {
 #[derive(Debug)]
 pub enum ErrorKind {
     FileError {
+        #[allow(dead_code)]
         source_path: PathBuf,
         source: io::Error,
     },
     IncludeNotFound {
+        #[allow(dead_code)]
         source_path: PathBuf,
     },
     ModuleNotFound {
+        #[allow(dead_code)]
         default_path: PathBuf,
+        #[allow(dead_code)]
         secondary_path: Option<PathBuf>,
     },
     ParserError {
+        #[allow(dead_code)]
         source_path: PathBuf,
         source: syn::Error,
+        #[allow(dead_code)]
         line: usize,
     },
 }