Add a helper method to strip subdirectories from URL paths

Problem/Motivation

In Url::toString() the UnroutedUrlAssembler::buildLocalUrl() method adds a the subdirectory to Drupal paths. The Url::fromUserInput() method only accepts URLs relative to the root directory though. This could lead to situations (for example action URLs in forms) where Drupal has a URL containing a subdirectory, but is unable to process them via Url::fromUserInput() because the URL is not a relative path. It would be nice to have a helper method to strip subdirectories from a path so you are able to feed the path to Url::fromUserInput().

See related issues:
#2990664: Media library does not work when Drupal is installed into a sub-directory →
#3002805: Media Widget Cannot Select Media - Page not Found/ReWrite Base Conflict →
#3042038: Allow UrlGenerator to process action URLs in FormBuilder::buildFormAction() →
🐛 \Drupal\Core\Url does not accept root-relative (file) URLs, making it impossible to let LinkGenerator create root-relative file URL links Active

Proposed resolution

Add a helper method to strip subdirectories from a path.

Remaining tasks

Write patch
Review
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User interface changes

None.

API changes

New helper method to strip subdirectories from a path.

Data model changes

None.

Release notes snippet

Problem/Motivation

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet