Update Makefile to use go.mod instead of relying on checkout location to infer project properties

[isutton]

Currently the Makefile extracts information from the project assuming a checkout in GOPATH:

# By default the project should be build under GOPATH/src/github.com/<orgname>/<reponame>
GO_PACKAGE_ORG_NAME ?= $(shell basename $$(dirname $$PWD))
GO_PACKAGE_REPO_NAME ?= $(shell basename $$PWD)
GO_PACKAGE_PATH ?= github.com/${GO_PACKAGE_ORG_NAME}/${GO_PACKAGE_REPO_NAME}

Extracting this information from the project's go.mod will use the same source of authority Go uses when building the project.

[pedjak]

Makefile got simplified in #848