fix resolution of imported css files

packages/esbuild-plugin-lit-css/esbuild-plugin-lit-css.ts

@@ -2,6 +2,7 @@ import type { Plugin } from 'esbuild';
 import type { Options } from '@pwrs/lit-css/lit-css';
 import { transform } from '@pwrs/lit-css';
 import { readFile } from 'node:fs/promises';
+import * as path from 'node:path';
 
 export interface LitCSSOptions extends Omit<Options, 'css'> {
   filter: RegExp;
@@ -13,12 +14,27 @@ export function litCssPlugin(options?: LitCSSOptions): Plugin {
     name: 'lit-css',
     setup(build) {
       const loader = 'js';
-      build.onLoad({ filter }, async args => {
+
+      build.onResolve({ filter }, args => {
+        return {
+          path: path.resolve(args.resolveDir, args.path),
+          namespace: 'env-lit-css',
+          pluginData: {
+            // Needed when using the "js" loader to properly resolve "lit"
+            resolveDir: args.resolveDir,
+          },
+        };
+      });
+
+      // Load paths tagged with the 'env-ns' namespace and behave as if
+      // they point to a JSON file containing the environment variables.
+      build.onLoad({ filter: /.*/, namespace: 'env-lit-css' }, async args => {
         const css = await readFile(args.path, 'utf8');
         const filePath = args.path;
+
         try {
           const contents = await transform({ css, specifier, tag, filePath, ...rest });
-          return { contents, loader };
+          return { contents, loader, resolveDir: args.pluginData.resolveDir };
         } catch (error) {
           return {
             errors: [{