chore: update directory structure and file names

[dgandhi62]

Changes include:

1. Renaming amplify-gen2-migration-codegen-dg directory to codegen-generate
2. Removing src from migration_head
3. Rename migration_head to codegen-head
4. Remove codegen/cli directory

The src directory needs to remain for the codegen-generate folder since there would be an additional tsconfig file added for codegen in the future. This is to enforce strict:false behavior in tsconfig as it was in the old tool. Currently, amplify-cli's tsconfig has been modified to make the option strict:false from strict:true (which needs to change back before merging to dev)

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

[kaizencc]

thanks! i'm still confused by these parts of the structure:

• src folder --> why is it necessary to have another src folder? the folder structure is now amplify-cli/src/comands/codegen-generate/src. src typically is a top-level folder that delineates where all the source code lives so it's confusing to me to see it again.
• migration_head folder -> what does this mean? why is this the head of the migration?
• cll folder -> are we releasing generate as a CLI separate from the migrate tool?
• npm_package folder -> are we releasing generate as a separate npm package?
• test_utils folder -> ok, but where are the tests? I can only see auth/index.test.ts. in the structure of the gen2-migration branch, i see src/__tests__/gen2-migration with some tests from Sai and Sanjana. i think that's where all the unit tests belong, and if so, that's where test_utils belong too if they are truly just utilities for tests

[dgandhi62]

1. The idea is that there needs to be a codegen-generate/tsconfig that applies for the entire codegen src files. Therefore, codegen-generate/src is what I figured makes sense. The codegen tsconfig needs to inherit from the amplify-cli one and override strict:false. I was facing errors while doing this and so I left it for before we merge to dev.

Although codegen is not its separate package, it is currently built to have some separate config requirements than amplify-cli.

2. The thought was that the entire codegen runs from "command_handler.ts" which lives in migration_head. Other than that, all the files with it were clumped together to begin with in a separate package in the old tool. Didn't think of separating them unless needed.

3. cli folder is unnecessary. This was there because I'd initially thought of integrating things differently before I saw the command files. Removing it.

4. npm package contains a singular file called "renderer.ts", which I believe helps in creating package.jsons. Its part of the migration process

5. The tests haven't been added yet. The unit tests in the old tool aren't comprehensive and I thought I'd go category by category and add them. The unit tests exist for auth-generator, the rest still need to be added / updated. I'm currently working on making the tests a little better for auth. Will add them soon.

I can make a separate pr to restructure the tests when they are ready en masse. Currently its just one file