The `go install` command doesn't like it when the go.mod file
declares a different path to the module, to the actual path.

e.g.

```
$ go install github.com/vercel/turborepo/cli/cmd/turbo@latest
go install: github.com/vercel/turborepo/cli/cmd/turbo@latest: github.com/vercel/turborepo/[email protected]: parsing go.mod:
	module declares its path as: turbo
	        but was required as: github.com/vercel/turborepo/cli
```

This PR renames the go module to match the path name, as go expects. This allows the turborepo CLI to be installed via `go install`