Overview
--------

This fixes a recent regression in which `xml/GenCode.php` fails to execute in certain configurations.

Initial report: https://github.com/civicrm/civicrm-buildkit/issues/503

Before
------

* If the folder `l10n` exists in its traditional location, and if you run `./bin/setup.sh -g`, then it
  correctly executes.
* If the folder `l10n` does not exist in its traditional locacation, and if you run `./bin/setup.sh -g`,
  then it fails with an error like this:
  ```
  + php -d mysql.default_host=127.0.0.1:3306 -d mysql.default_user=dmasterciv_abcde -d mysql.default_password=abcd1234 GenCode.php schema/Schema.xml '' Drupal

  civicrm_domain.version := 5.23.alpha1

  Parsing schema description schema/Schema.xml
  Extracting database information
  Extracting table information
  PHP Fatal error:  Uncaught RuntimeException: Invalid configuration: the [cms.root] path must be an absolute URL in /home/me/.../Civi/Core/Paths.php:174
  Stack trace:
  #0 /home/me/.../Civi/Core/Paths.php(151): Civi\Core\Paths->toAbsoluteUrl('/', 'cms.root')
  #1 /home/me/.../Civi/Core/Paths.php(176): Civi\Core\Paths->getVariable('cms.root', 'url')
  #2 /home/me/.../Civi/Core/Paths.php(151): Civi\Core\Paths->toAbsoluteUrl('/', 'civicrm.root')
  #3 /home/me/.../Civi/Core/Paths.php(224): Civi\Core\Paths->getVariable('civicrm.root', 'path')
  #4 /home/me/.../Civi/Core/Paths.php(84): Civi\Core\Paths->getPath('l10n')
  #5 [internal function]: Civi\Core\Paths->Civi\Core\{closure}()
  #6 /home/jo in /home/me/.../Civi/Core/Paths.php on line 174
  ```

After
-----

You can run `./bin/setup.sh -g` with or without the `l10n` folder.

Comments
--------

There's an argument to be made that this shouldn't be necessary: when running
`GenCode`, it should only create PHP files, and none of the output should depend
on the CMS URL - because that's liable to change when you deploy the PHP code.
If someone did try to generate URL at such an early stage, it's arguably good to
generate an error.  In point of fact, `GenCode` is not actually using the CMS
URL.

The oddity stems from the contract of `CRM_Utils_System_*` (specifically,
`getCiviSourceStorage()` and `getDefaultFileStorage()`) which do double-duty
providing both path and URL.  To avoid duplicate work, `Civi\Core\Paths` uses
the same grain of information - it tracks the pairs of path+URL.

A more aggressive fix might be to split `getCiviSourceStorage()` and
`getDefaultFileStorage()` so that it's possible to get the path and URL
separately; then revise `Civi\Core\Paths` to take advantage of the finer-grained
contract.  However, splitting those things would be a more invasive patch,
and we're currently in RC.