"lint" command for flipper-pkg tool

Summary:
Implemented json schema for flipper plugin package.json and used it for validation in "flipper-pkg lint" command.

Nice thing about json schema is that it not only allows to validate json, but also can be referenced using "$schema" property in json so IDEs like VSCode can find it and use for code completion, validation and to show properties documentation. I'm going to deploy the schema as a part of documentation website so it can be referenced as https://fbflipper.com/schemas/plugin-package/v2.json.

Also the "$schema" field can be used instead of "specVersion" to determine the specification according to which the plugin is defined. E.g., if specification version 3 would be created, it will be described in schema https://fbflipper.com/schemas/plugin-package/v3.json, etc.

Reviewed By: passy

Differential Revision: D21228294

fbshipit-source-id: f21351e584ef936a7d6b314436448489691f83a6

desktop/pkg/src/utils/validation/filePathExists.ts

@@ -0,0 +1,43 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ */
+
+import ajv from 'ajv';
+import path from 'path';
+import fs from 'fs-extra';
+
+export default function (inputDirectory: string) {
+  const filePathExists: ajv.KeywordDefinition = {
+    errors: true,
+    compile: function validatePathExists(schema: any) {
+      function filePathExistsValidator(value: any, dataPath: any) {
+        const it = filePathExistsValidator as ajv.SchemaValidateFunction;
+        if (!schema) {
+          return true;
+        }
+        it.errors = [];
+        const tpl = {
+          keyword: 'filePathExists',
+          dataPath,
+          schemaPath: '',
+          params: [],
+        };
+        const fullPath = path.resolve(inputDirectory, value);
+        if (!fs.pathExistsSync(fullPath) || !fs.lstatSync(fullPath).isFile()) {
+          it.errors.push({
+            ...tpl,
+            message: `should point to a valid file`,
+          });
+        }
+        return it.errors.length === 0;
+      }
+      return filePathExistsValidator;
+    },
+  };
+  return filePathExists;
+}