feat: Improve path resolution with aliases

This commit introduces a new function `getAbsPath` that improves the
resolution of file paths. It now supports absolute paths, relative paths,
and paths that start with an alias. The aliases are loaded from the
configuration. This change makes the code more robust and flexible.

Author: ryoppippi

mod.ts

@@ -1,3 +1,4 @@
+import process from "node:process";
 import * as path from "pathe";
 import * as fs from "node:fs/promises";
 import MagicString, { Bundle } from "magic-string";
@@ -7,7 +8,7 @@ import type { UserConfig as ViteConfig } from "vite";
 import { loadConfig } from "unconfig";
 
 async function loadAliases() {
-  const { config: { alias } } = await loadConfig({
+  const { config } = await loadConfig({
     merge: true,
     sources: [
       {
@@ -26,7 +27,35 @@ async function loadAliases() {
       },
     ],
   });
-  return alias;
+  return config?.alias ?? {};
+}
+
+/**
+ * Resolve paths
+ */
+async function getAbsPath(
+  { filename, file }: { filename?: string; file: string },
+) {
+  const aliases = await loadAliases();
+  const dirname = filename ? path.dirname(filename) : process.cwd();
+
+  if (file.startsWith("/")) {
+    return file;
+  }
+
+  if (file.startsWith("./") || file.startsWith("../")) {
+    return path.resolve(dirname, file);
+  }
+
+  for (const [alias, aliasPath] of Object.entries(aliases)) {
+    if (file.startsWith(alias)) {
+      const s = new MagicString(file);
+      s.overwrite(0, alias.length, aliasPath);
+      return path.resolve(s.toString());
+    }
+  }
+
+  return file;
 }
 
 /**
@@ -71,7 +100,7 @@ export function importCSSPreprocess(): PreprocessorGroup {
           } else {
             out.push(remove(0, end));
           }
-          const absPath = path.join(path.dirname(filename ?? ""), file);
+          const absPath = await getAbsPath({ filename, file });
           deps.push(absPath);
           const text = (await fs.readFile(absPath)).toString();
           out.push(new MagicString(text, { filename: absPath }));