Fix plugin API in globally installed Prettier and introduce optional --plugin-search-dir (#4192)

* Fix plugin API in globally installed Prettier and introduce optional --plugin-search-dir

* Use find-parent-dir instead of find-up and test autoloading (with mocked fn)

* Add two test cases where --plugin-search-dir is not .

* Do not mutate pluginSearchDirs argument in load-plugins.js

* Do not test automatic plugin resolution as mocking of "find-parent-dir" does not work due to rollup

* Document --plugin-search-dir / pluginSearchDirs and improve spacing

* Address @ikatyang's review comments

* Fix require path for third-party

* Undo alphabetic sorting of third-party scripts

tests_integration/plugins/automatic/node_modules/@prettier/plugin-foo/package.json

@@ -0,0 +1 @@
+{}

tests_integration/plugins/automatic/node_modules/prettier-plugin-bar/package.json

@@ -0,0 +1 @@
+{}