refactor: move api utility isNameUnique to core as isResourceNameUnique

[jhockett]

Description of changes

move api utility isNameUnique() to amplify-cli-core as isResourceNameUnique() since it's a common use-case throughout the CLI

Description of how you validated changes

• yarn test passes
• successfully added, updated, and removed an api manually
• added QGL api and attempted to add REST api with same name

? Provide a friendly name for your resource to be used as a label for this categ
ory in the project: api
>> A resource named 'api' already exists. Amplify resource names must be unique 
and are case-insensitive.

Checklist

• PR description included
• yarn test passes
• Tests are changed(https://github.com/aws-amplify/amplify-cli/blob/master/CONTRIBUTING.md#tests)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[attilah]

LGTM!