Set external: true when loading astro config (#11819)

Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>

.changeset/moody-waves-think.md

@@ -0,0 +1,11 @@
+---
+'astro': major
+---
+
+Updates the Astro config loading flow to ignore processing locally-linked dependencies with Vite (e.g. `npm link`, in a monorepo, etc). Instead, they will be normally imported by the Node.js runtime the same way as other dependencies from `node_modules`.
+
+Previously, Astro would process locally-linked dependencies which were able to use Vite features like TypeScript when imported by the Astro config file. 
+
+However, this caused confusion as integration authors may test against a package that worked locally, but not when published. This method also restricts using CJS-only dependencies because Vite requires the code to be ESM. Therefore, Astro's behaviour is now changed to ignore processing any type of dependencies by Vite.
+
+In most cases, make sure your locally-linked dependencies are built to JS before running the Astro project, and the config loading should work as before.

packages/astro/src/core/config/vite-load.ts

@@ -11,20 +11,7 @@ async function createViteServer(root: string, fs: typeof fsType): Promise<ViteDe
 		optimizeDeps: { noDiscovery: true },
 		clearScreen: false,
 		appType: 'custom',
-		ssr: {
-			// NOTE: Vite doesn't externalize linked packages by default. During testing locally,
-			// these dependencies trip up Vite's dev SSR transform. Awaiting upstream feature:
-			// https://github.com/vitejs/vite/pull/10939
-			external: [
-				'@astrojs/tailwind',
-				'@astrojs/mdx',
-				'@astrojs/react',
-				'@astrojs/preact',
-				'@astrojs/sitemap',
-				'@astrojs/markdoc',
-				'@astrojs/db',
-			],
-		},
+		ssr: { external: true },
 		plugins: [loadFallbackPlugin({ fs, root: pathToFileURL(root) })],
 	});