dr cache:rebuild, --url and DRUPAL_URI do not select the site in a multisite

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Problem/Motivation

On a multisite, dr uses sites/default in cases where another site was requested:

• dr cache:rebuild ignores --url, DRUPAL_URI and DRUPAL_DEV_SITE_PATH. It builds its request from globals.
• For all commands, --url and DRUPAL_URI never select a site directory. Request::create() leaves SCRIPT_NAME empty, so DrupalKernel::findSitePath() matches nothing.
• A URI without a scheme (--url=example.com, as in the option's help text) is parsed as a path.

Steps to reproduce

Install sites/default and sites/dev, add sites/sites.php, run dr cache:rebuild --url=https://dev. The caches of sites/default are rebuilt.

Proposed resolution

• Add DrupalApplication::getSitePath(), used by bootstrap() and cache:rebuild. It sets a script name from the URI path before calling findSitePath().
• drupal_rebuild() takes an optional $site_path.
• DrupalApplication::getUri() prepends http:// when the URI has no scheme.

API changes

drupal_rebuild() gains an optional third parameter.