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