Use ecs-agent/acs/model and ecs-agent/gogenerate

[danehlim]

Summary

Use ecs-agent/acs/model in favor of agent/acs/model and ecs-agent/gogenerate in favor of agent/gogenerate. This is a followup to #3643.

Implementation details

1. Use ecs-agent/acs/model instead of agent/acs/model
   • In agent/ directory, replace all imports of github.com/aws/amazon-ecs-agent/agent/acs/model/ecsacs with github.com/aws/amazon-ecs-agent/ecs-agent/acs/model/ecsacs
   • Resort imports in affected files
   • Delete agent/acs/model directory and all of its files
2. Use ecs-agent/gogenerate over agent/gogenerate
   • In agent/ directory, modify all go generate commands using ../../gogenerate/awssdk.go to use ../../../ecs-agent/gogenerate/awssdk.go instead
   • Delete agent/gogenerate directory and all of its files
   • Vendor github.com/aws/aws-sdk-go/private/model/api, github.com/aws/aws-sdk-go/private/util, and golang.org/x/tools/imports packages explicitly in agent/ since we need these to generate code as part of make gogenerate, but they are not automatically vendored after deleting agent/gogenerate directory and all of its files (i.e., the previous step).
     • We do this by importing them in agent/tools.go, which is the best practice for managing tool dependencies: cmd/go: clarify best practice for tool dependencies golang/go#25922 (comment). This is also explained in more detail in the previous pull request where we introduced the agent/tools.go file: migrate from dep to go module #2890.
   • Run go mod tidy and go mod vendor in agent/ directory

Testing

Unit, integration, and functional tests.
Manually ran make gogenerate to ensure it still succeeds after this change.

Description for the changelog

Use ecs-agent/acs/model and ecs-agent/gogenerate

Licensing

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

[YashdalfTheGray]

Tests are passing on this, the change itself looks alright. Good use of the tools file to prevent packages being cleaned up when running go mod tidy.