Drop support for godep, glide, dep, and buildpack detection by vendor folder

Go modules, Go's official solution to dependency management, have been on by default since Go 1.16. Per the docs and the output I see when running cf push, the buildpack still contains several dependency management solutions that predate modules:

• Godep, archived on Github since 2019
• Glide, not updated since 2019
• dep, deprecated and archived since 2020

Additionally, the buildpack checks for a vendor folder, but go modules does not vendor dependencies by default; instead, it downloads dependencies go $GOPATH/pkg/mod. Vendoring is still possible, but in practice, I see few projects do it.

I propose removing the old dependency management solutions in favor of Go modules, and propose detecting the Go buildpack by checking for the presence of a go.mod file.

Start by locating the Go buildpack's dependency-management and buildpack-detection entry points; the issue identifies vendor folders, Godep, Glide, dep, and go.mod as the relevant signals. Review the linked Cloud Foundry documentation and existing behavior before deciding the affected scope. Done means obsolete dependency detection is removed and go.mod-based detection is covered by the repository's existing tests.