fix: fall back to importing dynamic dependencies relative to SvelteKit package (#12532)

Simplifies code a bit around usage locations in our own code, and solves some issues people may have with their setups
closes #11578

---------

Co-authored-by: Ben McCann <322311+benmccann@users.noreply.github.com>
Co-authored-by: 오병진 <64676070+sunrabbit123@users.noreply.github.com>

Author: 3 people

.changeset/tricky-books-study.md

@@ -0,0 +1,5 @@
+---
+"@sveltejs/kit": patch
+---
+
+fix: fall back to importing dynamic dependencies relative to SvelteKit package

packages/kit/src/core/sync/utils.js

@@ -3,15 +3,8 @@ import path from 'node:path';
 import { mkdirp } from '../../utils/filesystem.js';
 import { resolve_peer_dependency } from '../../utils/import.js';
 
-/** @type {string} */
-let VERSION;
-
-try {
-	({ VERSION } = await resolve_peer_dependency('svelte/compiler'));
-} catch {
-	// we can end up here from e.g. unit tests. this is the simplest fix
-	({ VERSION } = await import('svelte/compiler'));
-}
+/** @type {{ VERSION: string }} */
+const { VERSION } = await resolve_peer_dependency('svelte/compiler');
 
 /** @type {Map<string, string>} */
 const previous_contents = new Map();

packages/kit/src/utils/import.js

@@ -4,14 +4,14 @@ import { pathToFileURL } from 'node:url';
 
 /**
  * Resolve a dependency relative to the current working directory,
- * rather than relative to this package
+ * rather than relative to this package (but falls back to trying that, if necessary)
  * @param {string} dependency
  */
-export function resolve_peer_dependency(dependency) {
+export async function resolve_peer_dependency(dependency) {
 	try {
 		// @ts-expect-error the types are wrong
 		const resolved = imr.resolve(dependency, pathToFileURL(process.cwd() + '/dummy.js'));
-		return import(resolved);
+		return await import(resolved).catch(() => import(dependency));
 	} catch {
 		throw new Error(
 			`Could not resolve peer dependency "${dependency}" relative to your project — please install it and try again.`