flag: replace regexp use

[kolyshkin]

What type of PR is this?

• cleanup

What this PR does / why we need it:

Commit ec05a8d introduces the only (non-test) user of regexp package.
It is pretty easy to not use regexp here, so let's get rid of it.

References:

• chore(deps): upgrade urfave/cli from v1 to v3 opencontainers/runc#5184 (comment)
• Do not use regexp opencontainers/runc#3460

Which issue(s) this PR fixes:

None

Special notes for your reviewer:

None

Testing

Assuming existing tests cover the change.

Release Notes

Remove regexp dependency.

[kolyshkin]

@dearchap PTAL

[dearchap]

@kolyshkin can you resolve the conflicts ?

[abitrolly]

 		// v1 -> v2 migration warning zone:
		// Strip off anything after the first found comma or space, which
		// *hopefully* makes it a tiny bit more obvious that unexpected behavior is
		// caused by using the v1 form of stringly typed "Name".

What all this comment is about? Is it still actual for v3?

[dearchap]

@abitrolly yes. Still valid.

[abitrolly]

@kolyshkin how did you trace size changes? Perhaps we could use a CI/CD bot to evaluate PRs.

[kolyshkin]

@kolyshkin how did you trace size changes?

See opencontainers/runc#5184 (comment) (about 500K)