Breaking: #108084 - Allow rootless paths in URI implementation 

Description 

Previously, the TYPO3 implementation of UriInterface always prefixed rootless paths (paths without preceding slash) with a slash. With this normalization in place is was impossible to represent rootless paths. This is now changed, that a slash is only prepended to the path, when an authority (host name) is present.

$uri = new \TYPO3\CMS\Core\Http\Uri('rootless/path/');
$uriAsString = (string)$uri;
// before: /rootless/path/
// after: rootless/path/

// Same behaviour with authority
$uri = (new \TYPO3\CMS\Core\Http\Uri('https://example.com))->withPath('rootless/path/');
$uriAsString = (string)$uri;
// before: https://example.com/rootless/path/
// after: https://example.com/rootless/path/

// Colon in first path segment
$uri = new \TYPO3\CMS\Core\Http\Uri('rootless:path/to/resource');
$uriAsString = (string)$uri;
// before: /rootless:path/to/resource/
// after: ./rootless:path/to/resource/

Impact 

Regarding top level TYPO3 API and functionality, nothing has changed. Required TYPO3 code has been adapted.

Third party code, that is using the Uri class directly will get different results when representing rootless paths without authority. Code that relies on the normalization done by TYPO3 before is likely to break.

Since TYPO3 is always dealing with absolute paths, due to URL rewriting in backend in frontend, likely not much third party code likely relies on relative paths, so the impact is expected to be low.

Affected installations 

Third party code, that is using the Uri class directly and that is representing rootless paths without authority.