add unknown package-ID in build-order

[memsharded]

Changelog: Bugfix: Include "Package ID Unknown" nodes in conan graph build-order, as they need to be processed in that order.
Docs: Omit

Close #6232

[jgsogo]

I feel like it is easier to see the JSON itself

[memsharded]

Fixed the test following to your suggestion.

Please feedback that it makes sense to include ID-Unknown packages in the build-order output.

[jgsogo]

I wouldn't add the :Package_ID_unknown to the reference in the JSON output, because:

>>> from conans.model.ref import ConanFileReference
>>> r = ConanFileReference.loads("liba/0.1#7086607aa6efbad8e2527748e3ee8237:Package_ID_unknown")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/jgsogo/dev/conan/conan/conans/model/ref.py", line 198, in loads
    ref = ConanFileReference(name, version, user, channel, revision, validate=validate)
  File "/Users/jgsogo/dev/conan/conan/conans/model/ref.py", line 172, in __new__
    obj._validate()
  File "/Users/jgsogo/dev/conan/conan/conans/model/ref.py", line 185, in _validate
    ConanName.validate_revision(self.revision)
  File "/Users/jgsogo/dev/conan/conan/conans/model/ref.py", line 147, in validate_revision
    "%s" % ConanName._max_chars)
conans.errors.InvalidNameException: The revision field, must contain only letters and numbers with a length between 1 and 51

Leaving it without the <package_id> part (liba/0.1#7086607aa6efbad8e2527748e3ee8237) means also unknown and it is parseable by the ConanFileReference.

[memsharded]

I have done the changes, but actually, the output references are PackageReference not ConanFileReference. It is parseable by PackageReference. Actually it will fail if it has an empty ID

[jgsogo]

🤦‍♂ Yes, PackageReference is able to parse them... and without it, it fails.