adjust the validation of mandatory options

[devoncarew]

• change the validation of mandatory options; perform validation when the value is retrieved (from the ArgResults object), instead of when the args are parsed
• fix use of 'mandatory' options breaks handling of the help flag #245

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[devoncarew]

For when we're ready to publish this (after sdk+google3 validation):

Package	Version	Status	Publish tag (post-merge)
package:args	2.4.2	ready to publish	v2.4.2

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.