Fix deepcopy/pickle of DAGCircuit variable IO nodes (backport #14041)

[mergify]

Summary

Fixes a typo we've had for a while in DAGCircuit::pack_into that would cause Var output nodes to become input nodes during deepcopy or pickling.

Details and comments

The DAGCircuit::pack_into method is only called with IO nodes from within DAGCircuit::__setstate__, so this bug would only trigger for deep-copied or un-pickled DAGs with variables.

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This is an automatic backport of pull request #14041 done by Mergify.

[mergify]

Cherry-pick of ddd0401 has failed:

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Changes to be committed:
	new file:   releasenotes/notes/fix-dag-var-out-pickle-a7d57e97a330e528.yaml
	modified:   test/python/dagcircuit/test_dagcircuit.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   crates/circuit/src/dag_circuit.rs

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[coveralls]

Pull Request Test Coverage Report for Build 13924716828

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 18 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.02%) to 88.073%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/lex.rs	6	91.73%
crates/qasm2/src/parse.rs	12	97.15%

Totals
Change from base Build 13904878030:	-0.02%
Covered Lines:	72684
Relevant Lines:	82527

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💛 - Coveralls