fix: resolve aliases before transpiling for rewriteRelativeImportExtensions (#14673)

This swaps the order of TypeScript transpilation and alias resolution in `@sveltejs/package` to fix #13044. TypeScript sees aliases like `$lib/` as bare imports and does not apply `rewriteRelativeImportExtensions`, and resolving aliases first seems like the correct fix. For more see microsoft/TypeScript#61991 (comment)

Author: ryanatkn

.changeset/slick-queens-laugh.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/package': patch
+---
+
+fix: resolve aliases before transpiling for `rewriteRelativeImportExtensions`

packages/package/src/index.js

@@ -224,11 +224,14 @@ async function process_file(input, output, file, preprocessor, aliases, tsconfig
 			}
 		}
 
+		// _before_ transpilining TS, that way TS will apply rewriteRelativeImportExtensions
+		// to these now-resolved path aliases, too
+		contents = resolve_aliases(input, file.name, contents, aliases);
+
 		if (file.name.endsWith('.ts') && !file.name.endsWith('.d.ts')) {
 			contents = await transpile_ts(tsconfig, filename, contents);
 		}
 
-		contents = resolve_aliases(input, file.name, contents, aliases);
 		analyse_code(file.name, contents);
 		write(dest, contents);
 	} else {

packages/package/test/fixtures/typescript-alias-rewrites/expected/helper.d.ts

@@ -0,0 +1 @@
+export declare const helper: () => string;

packages/package/test/fixtures/typescript-alias-rewrites/expected/helper.js

@@ -0,0 +1 @@
+export const helper = () => '';

packages/package/test/fixtures/typescript-alias-rewrites/expected/index.d.ts

@@ -0,0 +1,2 @@
+import { helper } from "./helper.ts";
+export { helper };

packages/package/test/fixtures/typescript-alias-rewrites/expected/index.js

@@ -0,0 +1,2 @@
+import { helper } from "./helper.js";
+export { helper };

packages/package/test/fixtures/typescript-alias-rewrites/package.json

@@ -0,0 +1,16 @@
+{
+	"name": "typescript-alias-rewrites",
+	"private": true,
+	"version": "1.0.0",
+	"description": "typescript package with extension rewrites to aliased import paths",
+	"type": "module",
+	"peerDependencies": {
+		"svelte": "^5.0.0"
+	},
+	"exports": {
+		".": {
+			"types": "./dist/index.d.ts",
+			"svelte": "./dist/index.js"
+		}
+	}
+}

packages/package/test/fixtures/typescript-alias-rewrites/src/lib/helper.ts

@@ -0,0 +1 @@
+export const helper = () => '';

packages/package/test/fixtures/typescript-alias-rewrites/src/lib/index.ts

@@ -0,0 +1,2 @@
+import { helper } from '$lib/helper.ts';
+export { helper };

packages/package/test/fixtures/typescript-alias-rewrites/svelte.config.js

@@ -0,0 +1,5 @@
+import preprocess from 'svelte-preprocess';
+
+export default {
+	preprocess: preprocess()
+};