fix(cli): fixed addPluginDirectory

[NakZMichael]

Fix to prevent adding a directory that is already included in pluginDirectories.

Description of changes

When I ran amplify plugin configure -> select scannable plugin directories -> select add , if I specified a directory that was already included, it was registered as a duplicate.

I thought this behavior is maybe undesirable.

Issue #, if available

Description of how you validated changes

I don't know how to test automatically a validator function in inquirer.prompt 😢

• manual test

Checklist

• [x] PR description included
• [x] yarn test passes
• [ ] Tests are changed or added
• [ ] Relevant documentation is changed or added (and PR referenced)

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