rename internal add_control to _add_control

[1ucian0]

Summary

The internal function add_control seems to create confusion among users. Here some examples:

• sympy2symengine error in circuit transpilation #13161
• Invalid Parameters with CU gate in AerSimulator #12625
• Running a specific quantum circuit multiple times gets different state vectors #11458
• https://stackoverflow.com/a/59545389/8146172
• https://stackoverflow.com/q/71108162/8146172

I think it can be renamed to mitigate that effect.

[qiskit-bot]

One or more of the following people are relevant to this code:

• @Qiskit/terra-core

[jakelishman]

I don't think we need to do this. It's not in the public interface, and I think changing its name like this risks suggesting that naming is what defines the public interface.

If we were going to do it, it'd probably be the module we renamed - the function itself is public within ourselves as a package (evidenced by how it's used in more than one file), and it's the module that's private to users.

[coveralls]

Pull Request Test Coverage Report for Build 10921247820

Details

• 3 of 3 (100.0%) changed or added relevant lines in 2 files are covered.
• 10 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.5%) to 88.829%

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Files with Coverage Reduction	New Missed Lines	%
crates/accelerate/src/two_qubit_decompose.rs	1	91.45%
crates/qasm2/src/lex.rs	3	92.73%
crates/qasm2/src/parse.rs	6	97.61%

Totals
Change from base Build 10919208944:	0.5%
Covered Lines:	73483
Relevant Lines:	82724

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💛 - Coveralls