Declare task output for validate and format tasks so they are incremental

[haydenbaker]

Background

• These tasks do not declare any outputs, therefore they are never considered up-to-date by gradle
• This change adds dummy outputs to these tasks
  • The real outputs are the same as the inputs, but marking the inputs as outputs becomes problematic when there are cross-project task dependencies

Testing

• Create a project that uses the smithy jar plugin
• Use a maven-local version, and run pTML with these changes
• Run smithyValidate -i, verify it (and smithyFormat) runs
• Run smithyValidate -i again, verify it (and smithyFormat) is marked up-to-date and not re-ran

Links

• Links to additional context, if necessary
• Issue #, if applicable (see here for a list of keywords to use for linking issues)

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