encoding/xml: treat a namespaced name as two names, not one

[DemiMarie]

An XML QName is syntactically two Names separated by a colon,
rather than a single name that has a colon in it. This fixes multiple
bugs in XML QName reading.

Fixes #68294
Fixes #68392
Fixes #68393

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Demi Obenour:

Patch Set 1:

(1 comment)

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