R1:328aee033fbd

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…