fix: SharedConfigPlugin uses closest shared config

### Before
If a page did not specify any shared config metadata then it would simply create a ref to the parent directory shared-config.json file.

However, this assumes that the parent directory *has* a shared config file.

### After
All generated shared config files are identified and then the closest 1 to the page is used.

packages/plugins/src/SharedConfigPlugin.ts

@@ -9,6 +9,16 @@ function createFileGlob(url, pageExtensions) {
   return `${url}{${pageExtensions.join(',')}}`;
 }
 
+/**
+ * https://stackoverflow.com/a/50549047
+ * compute inner relative to outer
+ * If it's not contained, the first component of the resulting path will be .., so that's what we check for
+ */
+function isWithin(outer, inner) {
+  const rel = path.posix.relative(outer, inner);
+  return !rel.startsWith('../') && rel !== '..';
+}
+
 interface SharedConfigPluginPage extends Page {
   sharedConfig?: string;
 }
@@ -35,8 +45,14 @@ const SharedConfigPlugin: PluginType<SharedConfigPluginPage, SharedConfigPluginO
       }
     );
 
+    const sharedConfigFiles = [];
+    const pagesWithoutConfig: string[] = [];
+
     for (const pagePath of pagePaths) {
-      const sharedConfigFile = path.posix.join(path.dirname(String(pagePath)), options.filename);
+      const sharedConfigFile = path.posix.join(
+        path.posix.dirname(String(pagePath)),
+        options.filename
+      );
 
       const page = await serialiser.deserialise(
         String(pagePath),
@@ -45,11 +61,36 @@ const SharedConfigPlugin: PluginType<SharedConfigPluginPage, SharedConfigPluginO
       if (page.sharedConfig) {
         config.setRef(sharedConfigFile, ['config', '$ref'], `${String(pagePath)}#/sharedConfig`);
         await mutableFilesystem.promises.writeFile(sharedConfigFile, '{}');
+        sharedConfigFiles.push(sharedConfigFile);
       } else {
-        const baseDir = path.posix.resolve(path.dirname(String(pagePath)), '../');
+        pagesWithoutConfig.push(page.fullPath);
+      }
+    }
+
+    let closestSharedConfigIndex = 0;
+    for (const pagePath of pagesWithoutConfig) {
+      for (let i = 0; i < sharedConfigFiles.length; i++) {
+        const sharedConfigBaseDir = path.posix.resolve(
+          path.posix.dirname(sharedConfigFiles[i]),
+          '../'
+        );
+        const pageBaseDir = path.posix.resolve(path.posix.dirname(String(pagePath)), '../');
 
-        config.setAliases(path.join(baseDir, options.filename), [sharedConfigFile]);
+        if (isWithin(sharedConfigBaseDir, pageBaseDir)) {
+          closestSharedConfigIndex = i;
+        }
       }
+
+      const sharedConfigFile = path.posix.join(
+        path.posix.dirname(String(pagePath)),
+        options.filename
+      );
+
+      const closestSharedConfig = path.posix.resolve(
+        path.dirname(String(pagePath)),
+        sharedConfigFiles[closestSharedConfigIndex]
+      );
+      config.setAliases(closestSharedConfig, [sharedConfigFile]);
     }
   }
 };