Fix base prefix when generating URLs if index.php is in the path

Problem/Motivation

Follow-up to #1088112: Introduce a token to get site's base URL

Steps to reproduce:

1. Go to your installed drupal directory and append index.php (e.g http://drupal.test/index.php)
2. Build a URL with base prefix: Url::fromUri('base:sites/default/files/og_image.jpg')
3. Assert the URL to the image asset is broken as the path is /index.php/sites/default/files/og_image.jpg

Proposed resolution

When building a URL with base prefix use $request->getBasePath() instead of $request->getBaseUrl() which results in omitting index.php in case it is present.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet