Avoid uncaught exception when file is deleted (#3313)

If I run `snowpack dev` and then delete a watched file, the process crashes with this error:

  ⠸ watching for file changes...node:internal/process/promises:246
            triggerUncaughtException(err, true /* fromPromise */);
            ^
  [Error: ENOENT: no such file or directory, open '/home/ahupp/new-dir/src/index.js'] {
    errno: -2,
    code: 'ENOENT',
    syscall: 'open',
    path: '/home/ahupp/new-dir/src/index.js'
  }

The root issue seems to be that 'unlink' events go through the same
path as change/add events, which try to read the file and throw an
uncaught exception.  This diff moves the call to `prepareSingleFile`
so that it only runs in the change and add paths.

I'm not very familiar with this codebase so feel free to tell me this
is the wrong approach, but it does fix the problem for me.

This should fix #3252

Other issues to consider:

 * The stack trace for these exceptions seems to be lost, which makes
   debugging this more challenging than necessary.
 * Should exceptions from chokidar watch events be caught and logged?

snowpack/src/commands/dev.ts

@@ -926,11 +926,11 @@ export async function startServer(
 
   // Watch src files
   async function onWatchEvent(fileLoc: string) {
+
     logger.info(
       colors.cyan('File changed: ') + path.relative(config.workspaceRoot || config.root, fileLoc),
     );
-    // TODO: If this needs to build a new dependency, report to the browser via HMR event.
-    await pkgSource.prepareSingleFile(fileLoc);
+
     const updatedUrls = getUrlsForFile(fileLoc, config);
     if (updatedUrls) {
       handleHmrUpdate && handleHmrUpdate(fileLoc, updatedUrls[0]);
@@ -940,6 +940,7 @@ export async function startServer(
     inMemoryBuildCache.delete(getCacheKey(fileLoc, {isSSR: true, mode: config.mode}));
     inMemoryBuildCache.delete(getCacheKey(fileLoc, {isSSR: false, mode: config.mode}));
     await onFileChangeCallback({filePath: fileLoc});
+
     for (const plugin of config.plugins) {
       plugin.onChange && plugin.onChange({filePath: fileLoc});
     }
@@ -956,18 +957,21 @@ export async function startServer(
       disableGlobbing: false,
       useFsEvents: isFsEventsEnabled(),
     });
-    watcher.on('add', (fileLoc) => {
+    watcher.on('add', async (fileLoc) => {
       knownETags.clear();
-      onWatchEvent(fileLoc);
+      await pkgSource.prepareSingleFile(fileLoc);
+      await onWatchEvent(fileLoc);
       fileToUrlMapping.add(fileLoc, getUrlsForFile(fileLoc, config)!);
     });
-    watcher.on('unlink', (fileLoc) => {
+    watcher.on('unlink', async (fileLoc) => {
       knownETags.clear();
-      onWatchEvent(fileLoc);
+      await onWatchEvent(fileLoc);
       fileToUrlMapping.delete(fileLoc);
     });
-    watcher.on('change', (fileLoc) => {
-      onWatchEvent(fileLoc);
+    watcher.on('change', async (fileLoc) => {
+      // TODO: If this needs to build a new dependency, report to the browser via HMR event.
+      await pkgSource.prepareSingleFile(fileLoc);
+      await onWatchEvent(fileLoc);
     });
     // [hmrDelay] - Let users with noisy startups delay HMR (ex: 11ty, tsc builds)
     setTimeout(() => {