User plugin foundations for 0.6.0 (#257)

Starts work on #55.

This is similar to the previous work in #125. It's gated behind a new Cargo
feature, `user-plugins`. This time, the config gate is much smaller. The
`plugins` member of projects is still accessible when plugins aren't enabled,
but is always empty. Additionally, user plugins are only enabled if there's a
Lua state present in the snapshot context when the `SnapshotUserPlugins`
snapshot middleware runs. This not ever the case currently.

This code has very little possibility of rotting while we focus on other work,
since it'll be guaranteed to still compile and can be tested in isolation
without the feature being turned on.

src/project.rs

@@ -30,6 +30,10 @@ struct SourceProject {
 
     #[serde(skip_serializing_if = "Option::is_none")]
     serve_place_ids: Option<HashSet<u64>>,
+
+    #[serde(default, skip_serializing_if = "Vec::is_empty")]
+    #[cfg_attr(not(feature = "user-plugins"), serde(skip_deserializing))]
+    plugins: Vec<String>,
 }
 
 impl SourceProject {
@@ -37,11 +41,19 @@ impl SourceProject {
     pub fn into_project(self, project_file_location: &Path) -> Project {
         let tree = self.tree.into_project_node(project_file_location);
 
+        let project_folder = project_file_location.parent().unwrap();
+        let plugins = self
+            .plugins
+            .into_iter()
+            .map(|path| project_folder.join(path))
+            .collect();
+
         Project {
             name: self.name,
             tree,
             serve_port: self.serve_port,
             serve_place_ids: self.serve_place_ids,
+            plugins,
             file_location: PathBuf::from(project_file_location),
         }
     }
@@ -318,6 +330,7 @@ pub struct Project {
     pub tree: ProjectNode,
     pub serve_port: Option<u16>,
     pub serve_place_ids: Option<HashSet<u64>>,
+    pub plugins: Vec<PathBuf>,
     pub file_location: PathBuf,
 }
 
@@ -391,6 +404,7 @@ impl Project {
             tree,
             serve_port: None,
             serve_place_ids: None,
+            plugins: Vec::new(),
             file_location: project_path.clone(),
         };
 
@@ -557,10 +571,24 @@ impl Project {
     }
 
     fn to_source_project(&self) -> SourceProject {
+        // TODO: Use path_serializer instead of transforming paths between
+        // String and PathBuf?
+        let plugins = self
+            .plugins
+            .iter()
+            .map(|path| {
+                path.strip_prefix(self.folder_location())
+                    .unwrap()
+                    .display()
+                    .to_string()
+            })
+            .collect();
+
         SourceProject {
             name: self.name.clone(),
             tree: self.tree.to_source_node(&self.file_location),
             serve_port: self.serve_port,
+            plugins,
             serve_place_ids: self.serve_place_ids.clone(),
         }
     }