Move cmd to cmd/cloudprober for simplified install

[l2dy]

Describe the feature you'd like and the problem it will solve

Currently the Getting Started guide gives 2 commands to install cloudprober and requires GOPATH, which is often not set in recent Go environments that uses Go modules.

go get github.com/cloudprober/cloudprober
GOBIN=$GOPATH/bin go install $GOPATH/src/github.com/cloudprober/cloudprober/cmd/cloudprober.go

I propose that we move cmd to cmd/cloudprober, so that the installation instructions can be simplified to

go install github.com/cloudprober/cloudprober/cmd/cloudprober@latest

and does not require the GOPATH environment variable anymore.

[manugarg]

Thanks @l2dy for reporting this. It makes sense but I'll need to change things in a bunch of places for this. I'll take a look at it.