Move to using go modules

[dastra]

Issue #, if available:

#23

Description of changes:

The carts, orders, users services use go modules instead of getting the latest version. Pinned to the latest versions available today.

Description of testing performed to validate your changes (required if pull request includes CloudFormation or source code changes):

Ran the services locally in docker, ran the integration tests, tested by browsing the website.

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