fix(next-swc/relay): make pages directory optional (#43116)

Fixes:
relayjs/relay-examples#241 (comment)

**Context:**
Current relay transformer requires `pages_dir` in its entry point:

https://github.com/vercel/next.js/blob/ecfd2f4cd6a1c3f6bc3cd1cca6b62a3cb705a4e2/packages/next-swc/crates/core/src/relay.rs#L184

But consider the case that `pages_dir` is not provided because the page
system is built entirely in the app directory introduced from Next.js
13.

In this case, transformer causes unconditional panics even though
there's nothing wrong with it.

This PR removes panic in entry point and changes the type of `pages_dir`
into `Option<T>` so that keeps [build stability for existing page
system](https://github.com/vercel/next.js/blob/ecfd2f4cd6a1c3f6bc3cd1cca6b62a3cb705a4e2/packages/next-swc/crates/core/src/relay.rs#L157-L160)
even if `pages_dir` is not provided.

packages/next-swc/crates/core/src/relay.rs

@@ -27,7 +27,7 @@ impl Default for RelayLanguageConfig {
 
 struct Relay<'a> {
     root_dir: PathBuf,
-    pages_dir: PathBuf,
+    pages_dir: Option<PathBuf>,
     file_name: FileName,
     config: &'a Config,
 }
@@ -107,7 +107,11 @@ impl<'a> Relay<'a> {
 
         if let Some(artifact_directory) = &self.config.artifact_directory {
             Ok(self.root_dir.join(artifact_directory).join(filename))
-        } else if real_file_name.starts_with(&self.pages_dir) {
+        } else if self
+            .pages_dir
+            .as_ref()
+            .map_or(false, |pages_dir| real_file_name.starts_with(pages_dir))
+        {
             Err(BuildRequirePathError::ArtifactDirectoryExpected {
                 file_name: real_file_name.display().to_string(),
             })
@@ -181,7 +185,7 @@ pub fn relay(config: &Config, file_name: FileName, pages_dir: Option<PathBuf>) -
     Relay {
         root_dir: std::env::current_dir().unwrap(),
         file_name,
-        pages_dir: pages_dir.unwrap_or_else(|| panic!("pages_dir is expected.")),
+        pages_dir,
         config,
     }
 }