refactor: rename package module loading to module directory loading in readModule

Rename the internal concept from "package" to "module directory" when loading
module.wren files from subdirectories. Update variable names (packageModule
to moduleDir, packageModulePath to moduleDirPath) and adjust early return
logic to free modulePath before checking contents. Add test for loading
module from a directory and remove old package module test.

src/cli/io.c

@@ -74,29 +74,24 @@ char* readModule(WrenVM* vm, const char* module)
   // First try to load the module with a ".wren" extension.
   char* modulePath = wrenFilePath(module);
   char* moduleContents = readFile(modulePath);
+  free(modulePath);
+
+  if (moduleContents != NULL) return moduleContents;
 
   // If no contents could be loaded treat the module name as specifying a
-  // package and try to load the "module.wren" file in the package.
-  if (moduleContents == NULL)
-  {
-    free(modulePath);
+  // directory and try to load the "module.wren" file in the directory.
+  size_t moduleLength = strlen(module);
+  size_t moduleDirLength = moduleLength + 7;
+  char* moduleDir = (char*)malloc(moduleDirLength + 1);
+  memcpy(moduleDir, module, moduleLength);
+  memcpy(moduleDir + moduleLength, "/module", 7);
+  moduleDir[moduleDirLength] = '\0';
 
-    size_t moduleLength = strlen(module);
-    size_t modulePackageLength = moduleLength + 7;
-    char* packageModule = (char*)malloc(modulePackageLength + 1);
-    memcpy(packageModule, module, moduleLength);
-    memcpy(packageModule + moduleLength, "/module", 7);
-    packageModule[modulePackageLength] = '\0';
+  char* moduleDirPath = wrenFilePath(moduleDir);
+  free(moduleDir);
 
-    char* packageModulePath = wrenFilePath(packageModule);
-    moduleContents = readFile(packageModulePath);
-
-    if (moduleContents == NULL)
-    {
-      free(packageModulePath);
-      return NULL;
-    }
-  }
+  moduleContents = readFile(moduleDirPath);
+  free(moduleDirPath);
 
   return moduleContents;
 }

test/language/module/module_dir/module_dir.wren

@@ -0,0 +1,3 @@
+import "something" for Index
+
+IO.print(Index) // expect: index

test/language/module/package_module.wren

@@ -1,3 +0,0 @@
-import "module_package" for Index
-
-IO.print(Index) // expect: index