fix(deps): update dependency flat to v6 (#579)

* fix(deps): update dependency flat to v6

* Change import of flatten

* Use import for flatten

Resolves error:

`Error [ERR_REQUIRE_ESM]: require() of ES Module /home/runner/work/google-cloudevents/google-cloudevents/tools/proto2jsonschema/node_modules/flat/index.js from /home/runner/work/google-cloudevents/google-cloudevents/tools/proto2jsonschema/postgen.js not supported.
Instead change the require of index.js in /home/runner/work/google-cloudevents/google-cloudevents/tools/proto2jsonschema/postgen.js to a dynamic import() which is available in all CommonJS modules.`

* Undo changes to postgen.js

* Use import in postgen.js

* set type to module package.json

* resolve __dirname error

ReferenceError: __dirname is not defined in ES module scope

---------

Co-authored-by: Roger Martinez <31829545+rogerthatdev@users.noreply.github.com>

tools/proto2jsonschema/package.json

@@ -4,8 +4,9 @@
     "postgen": "node postgen.js"
   },
   "dependencies": {
-    "flat": "^5.0.2",
+    "flat": "^6.0.0",
     "protobufjs": "^7.0.0",
     "recursive-readdir": "^2.2.2"
-  }
+  },
+  "type": "module"
 }

tools/proto2jsonschema/postgen.js

@@ -14,11 +14,11 @@
  * limitations under the License.
  */
 
-const path = require('path');
-const fs = require('fs');
-const recursive = require("recursive-readdir");
-const protobufjs = require('protobufjs');
-const flatten = require('flat');
+import path from 'path';
+import fs from 'fs';
+import recursive from 'recursive-readdir';
+import protobufjs from 'protobufjs';
+import { flatten } from 'flat'
 
 /**
  * This tool polishes the JSON schemas with a few modifications:
@@ -27,6 +27,8 @@ const flatten = require('flat');
  * - Adds "examples" - A list of paths to the test event data associated with the schema
  *   - e.g. ["https://googleapis.github.io/google-cloudevents/testdata/google/events/cloud/audit/v1/LogEntryData-pubsubCreateTopic.json"]
  */
+const __filename = new URL(import.meta.url).pathname;
+const __dirname = path.dirname(__filename);
 const ROOT = path.resolve(`${__dirname}/../../jsonschema`);
 const TESTDATA = path.resolve(`${__dirname}/../../testdata`);
 console.log(`Fixing paths in dir: ${ROOT}`);