Switch to using Go modules.

[paddycarver]

This migrates our vendor.json to use Go 1.11's modules system, and
replaces the vendor folder with the output of go mod vendor.

The vendored code should remain basically the same; I believe some
tree shaking of packages and support scripts/licenses/READMEs/etc.
happened.

This also fixes Travis and our Makefile to no longer use govendor.

(This is a replacement for #2666, except based on @ndmckinley's
work to get 2.0.0 stuff on master.)

[nat-henderson]

Delightful. :) Thank you!

[paddycarver]

To keep this here as well:

How this happened:

1. go mod init github.com/terraform-providers/terraform-provider-google-beta - initialize go modules, convert our old vendor.json.
2. go mod tidy - clean up the dependencies and find indirect dependencies.
3. go get github.com/hashicorp/terraform/helper/schema@35d82b055591e9d47a254e68754216d8849ba67a to vendor the version of helper/schema we need.
4. go mod tidy to clean up the vendoring again.
5. rm -r vendor/ to get rid of the old vendored code
6. go mod vendor to vendor based on Go modules
7. manually delete some lines and change some others in the Makefile

To vendor new things or update existing vendors once this is merged:

1. go get $url@$commit_sha_2_vendor ($commit_sha_2_vendor could also be a version, if the package is a module and support versions, and the commit we need is in a published version)
2. go mod tidy
3. go mod vendor
4. commit and PR

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you feel this issue should be reopened, we encourage creating a new issue linking back to this one for added context. If you feel I made an error 🤖 🙉 , please reach out to my human friends 👉 [email protected]. Thanks!