Move files to the sweeper and envvar packages

[modular-magician]

Main changes are in the files

• mmv1/third_party/terraform/acctest/gcp_sweeper.go
• mmv1/third_party/terraform/sweeper/gcp_sweeper.go
• mmv1/third_party/terraform/acctest/provider_test_utils.go
• mmv1/third_party/terraform/envvar/envvar_utils.go

In this PR,

1. Move the file gcp_sweeper.go from acctest package to the sweeper package
2. Move most functions in the file provider_test_utils.go in the acctest package to the package envvar
3. Replace the function calls.

The reason to introduce the new packages sweep and envvar is to avoid the import circle.

The testAccProvider will be moved to the acctest package from the google package later.

If the file gcp_sweeper.go and the env variables are still in the acctest package, the sweeper file in the specific service package imports the acctest package to add the sweeper to the sweepers map. The acctest package imports the provider package to initialize the testAccProvider. The provider package imports the service package. Then the import circle is formed.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
• Ran relevant acceptance tests using my own Google Cloud project and credentials (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read Write release notes before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

Derived from GoogleCloudPlatform/magic-modules#8198