isSelfURI: fix anchors and relative URIs (that are "self" indeed)

Summary:
Example cases that were wrongly considered external URLs:

- "#foo"
- "/foo"

Note that Phorge usually does not change stable things. In fact this thing
was not documented even inline, and was not even in the unit tests.

So this is a fix, and not a breaking change. Probably no one had ever tried it.

Closes T15182

Test Plan:
The already existing unit tests still work.

The added ones make sense.

There are no problems doing other random things.

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15182

Differential Revision: https://we.phorge.it/D25555

src/infrastructure/env/PhabricatorEnv.php

@@ -433,7 +433,7 @@ final class PhabricatorEnv extends Phobject {
 
     $host = $uri->getDomain();
     if (!phutil_nonempty_string($host)) {
-      return false;
+      return true;
     }
 
     $host = phutil_utf8_strtolower($host);

src/infrastructure/env/__tests__/PhabricatorEnvTestCase.php

@@ -243,6 +243,12 @@ final class PhabricatorEnvTestCase extends PhabricatorTestCase {
       'https://old.example.com/path/to/resource.png' => true,
 
       'https://other.example.com/' => false,
+
+      '/' => true,
+      '/self' => true,
+      '#self' => true,
+      '/#self' => true,
+      '/self/#self' => true,
     );
 
     foreach ($map as $input => $expect) {