fix: always ensure the base command is set to "sam"

[sriram-mv]

• irrespective of the module name that invokes the CLI, the command that
  is seen on the help/description text, should always say "sam"

Example

(aws_sam_cli_WS) srirammv@localrainbow:20:20:34:~/aws_sam_cli_WS/aws-sam-cli$python -m samcli
Usage: sam [OPTIONS] COMMAND [ARGS]...

  AWS Serverless Application Model (SAM) CLI

  The AWS Serverless Application Model extends AWS CloudFormation to provide
  a simplified way of defining the Amazon API Gateway APIs, AWS Lambda
  functions, and Amazon DynamoDB tables needed by your serverless
  application. You can find more in-depth guide about the SAM specification
  here: https://github.com/awslabs/serverless-application-model.

Options:
  --debug    Turn on debug logging to print debug message generated by SAM
             CLI.
  --version  Show the version and exit.
  --info
  --help     Show this message and exit.

Commands:
  publish   Publish a packaged AWS SAM template to the AWS Serverless
            Application Repository.
  package   Package an AWS SAM application. This is an alias for 'aws
            cloudformation package'.
  init      Initialize a serverless application with a...
  build     Build your Lambda function code
  validate  Validate an AWS SAM template.
  local     Run your Serverless application locally for...
  logs      Fetch logs for a function
  deploy    Deploy an AWS SAM application. This is an alias for 'aws
            cloudformation deploy'.

we now see sam [OPTIONS] COMMAND [ARGS]... instead of __main__.py [OPTIONS] COMMAND [ARGS]...
By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[sriram-mv]

Original make_context function: https://github.com/pallets/click/blob/38a2712d596a5df9288c1817748b71197d8c7c57/click/core.py#L621

[sriram-mv]

pallets/click#82 is a better way to do it.

[jfuss]

That is super clean. Nice find!

[sriram-mv]

restarting build, due to transient failure.