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

The already existing unit tests still work.

The added ones make sense.

There are no problems doing other random things.