dirmgr: NetworkConfig: Rename fallbacks struct field

Rename this to be named the same as its accessor, and use the rename
feature of builder and serde.  (The shorter name is nicer in the code
IMO.)

crates/tor-dirmgr/src/config.rs

@@ -40,7 +40,10 @@ pub struct NetworkConfig {
     /// affect future download attempts only.
     #[serde(default = "fallbacks::default_fallbacks")]
     #[builder(default = "fallbacks::default_fallbacks()")]
-    fallback_caches: Vec<FallbackDir>,
+    #[serde(rename = "fallback_caches")]
+    #[builder_field_attr(serde(rename = "fallback_caches"))]
+    #[builder(setter(name = "fallback_caches"))]
+    fallbacks: Vec<FallbackDir>,
 
     /// List of directory authorities which we expect to sign consensus
     /// documents.
@@ -57,7 +60,7 @@ pub struct NetworkConfig {
 impl Default for NetworkConfig {
     fn default() -> Self {
         NetworkConfig {
-            fallback_caches: fallbacks::default_fallbacks(),
+            fallbacks: fallbacks::default_fallbacks(),
             authorities: crate::authority::default_authorities(),
         }
     }
@@ -74,14 +77,14 @@ impl NetworkConfig {
     }
     /// Return the configured fallback directories
     pub(crate) fn fallbacks(&self) -> &[FallbackDir] {
-        &self.fallback_caches[..]
+        &self.fallbacks[..]
     }
 }
 
 impl NetworkConfigBuilder {
     /// Check that this builder will give a reasonable network.
     fn validate(&self) -> std::result::Result<(), ConfigBuildError> {
-        if self.authorities.is_some() && self.fallback_caches.is_none() {
+        if self.authorities.is_some() && self.fallbacks.is_none() {
             return Err(ConfigBuildError::Inconsistent {
                 fields: vec!["authorities".to_owned(), "fallbacks".to_owned()],
                 problem: "Non-default authorities are use, but the fallback list is not overridden"
@@ -252,7 +255,7 @@ impl DirMgrConfig {
         DirMgrConfig {
             cache_path: self.cache_path.clone(),
             network_config: NetworkConfig {
-                fallback_caches: new_config.network_config.fallback_caches.clone(),
+                fallbacks: new_config.network_config.fallbacks.clone(),
                 authorities: self.network_config.authorities.clone(),
             },
             schedule_config: new_config.schedule_config.clone(),
@@ -356,7 +359,7 @@ mod test {
         let mut bld = NetworkConfig::builder();
         let cfg = bld.build().unwrap();
         assert_eq!(cfg.authorities().len(), dflt.authorities.len());
-        assert_eq!(cfg.fallbacks().len(), dflt.fallback_caches.len());
+        assert_eq!(cfg.fallbacks().len(), dflt.fallbacks.len());
 
         // with any authorities set, the fallback list _must_ be set
         // or the build fails.