Fix required Bash flag completion

[giri256]

Fixes #2465.

Required value flags are emitted into the Bash completion state with an equals
suffix, such as --config=. The existing handler canonicalized
--config=value to that spelling, but compared two-word input as --config,
so the required set remained active.

This derives a separate name only for the required-set comparison when the
flag is a known two-word flag. The original flag name remains unchanged for
flag values, local flags, and argument skipping.

The regression executes the generated Bash script and covers:

• --required=value;
• --required value;
• an unrelated optional value flag;
• a required boolean flag;
• a required shorthand value flag.

Validation:

• go test ./...
• go vet ./...
• gofmt
• git diff --check

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