Add missing free of the old rootDirectory pointer in setRootDirectory to prevent
memory leaks when the function is called multiple times. Also set rootDirectory
to NULL via setRootDirectory after VM teardown in runFile to ensure the pointer
is properly cleared before program exit.
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.
Extract file path construction into `wrenFilePath` helper and modify `readModule` to first attempt loading the module as a `.wren` file. If that fails, treat the module name as a package directory and try to load `module.wren` from within it. Add test files `module_package/module.wren` and `package_module.wren` to verify the new package resolution behavior.
Extract the static `readFile`, `readModule`, and `setRootDirectory` functions from main.c into a new io module, replacing the fixed-size `rootDirectory` array with a heap-allocated pointer and adding the corresponding header with public declarations.
