fix: findExisting should be case sensitive

closes #2305

We have added `case-sensitive-paths-webpack-plugin` to the base config.
So the filename of instant prototyping entry should also be case-sensitive.

packages/@vue/cli-service-global/lib/util.js

@@ -3,9 +3,31 @@ const path = require('path')
 
 exports.toPlugin = id => ({ id, apply: require(id) })
 
+// Based on https://stackoverflow.com/questions/27367261/check-if-file-exists-case-sensitive
+// Case checking is required, to avoid errors raised by case-sensitive-paths-webpack-plugin
+function fileExistsWithCaseSync (filepath) {
+  const dir = path.dirname(filepath)
+
+  if (dir === '/' || dir === '.') {
+    return true
+  }
+
+  try {
+    const filenames = fs.readdirSync(dir)
+    if (!filenames.includes(path.basename(filepath))) {
+      return false
+    }
+  } catch (e) {
+    // dir does not exist
+    return false
+  }
+
+  return fileExistsWithCaseSync(dir)
+}
+
 exports.findExisting = (context, files) => {
   for (const file of files) {
-    if (fs.existsSync(path.join(context, file))) {
+    if (fileExistsWithCaseSync(path.join(context, file))) {
       return file
     }
   }