internal/flags: condense case/switch statement

[axelKingsley]

First noticed in #28692

The switch statement in flags.go uses a duplicate imperative for each case. This PR removes the duplicate code per case and merges the various flag types into a single case.

[fjl]

This is not possible in Go. Would be nice if it was, but it's not.

[axelKingsley]

Augh, you're right, thank you for looking at this. Wanted to leave a bit of detail here in case anyone sees this thread and wants to understand why this isn't possible.

In golang, it is possible to group multiple cases together and give them a single imperative: https://go.dev/play/p/cmN3qVOw31b

However, this is not possible in this specific case because this switch uses flag.(type), a special assertion for switch statements. Casting flag := flag.(type) is important because the shared interface cli.flag does not include EnvVars. However, each concrete type (StringFlag, BoolFlag, etc) does have an EnvVars.

My implementation, grouping up the types as a single case will not work because in that situation, the underlying object we're switching on hasn't been cast into any concrete types, and is just a cli.flag without an EnvVars.