datasource new overwrites existing datasources with the same name

[spbail]

Describe the bug
Creating a new datasource with the CLI overwrites existing datasources with the same name without warning.

To Reproduce
Steps to reproduce the behavior:

1. Run great_expectations datasource new and create a new filesystem datasource e.g. "data__dir", pointing to a directory "dir1"
2. Do the same thing again, give it the same name but point it at "dir2", and check great_expectations.yml - only the second datasource is in the yml.

Expected behavior
I would expect a warning or a confirmation that I'm overwriting an existing datasource.

Environment (please complete the following information):

• OS: mac OS 11 (Big Sur)
• GE Version: GE 0.13 (but I assume this exist in previous versions too)

[thcidale0808]

@eugmandel can I implement this issue?

[eugmandel]

@thcidale0808 Of course! This is why we added the "help wanted" label. Please submit a PR and we will help you get it in.

[thcidale0808]

will do! Thanks!

[alfredo-f]

It seems like function check_if_datasource_name_exists has been implemented to solve this issue.

Is this correct @anthonyburdi ?

[github-actions]

Is this still relevant? If so, what is blocking it? Is there anything you can do to help move it forward?\n\nThis issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[anthonyburdi]

That is correct, thanks for the heads up @alfredo-f !