Move pkg to cmd/nvidia-container-toolkit

This change moves the pkg folder to `cmd/nvidia-container-toolkit` to
better match go best practices. This allows, for example, for the
`cmd/nvidia-container-toolkit` to be go installed.

The only package included in `pkg` was `main`.

Signed-off-by: Evan Lezar <elezar@nvidia.com>

Makefile

@@ -21,17 +21,16 @@ LIB_VERSION := 1.5.0
 LIB_TAG ?=
 
 GOLANG_VERSION := 1.16.3
-GOLANG_PKG_PATH := github.com/NVIDIA/nvidia-container-toolkit/pkg
+MODULE := github.com/NVIDIA/nvidia-container-toolkit
 
 # By default run all native docker-based targets
 docker-native:
 include $(CURDIR)/docker/docker.mk
 
 binary:
-	go build -ldflags "-s -w" -o "$(LIB_NAME)" $(GOLANG_PKG_PATH)
+	go build -ldflags "-s -w" -o "$(LIB_NAME)" $(MODULE)/cmd/$(LIB_NAME)
 
 # Define the check targets for the Golang codebase
-MODULE := .
 .PHONY: check fmt assert-fmt ineffassign lint misspell vet
 check: assert-fmt lint misspell vet
 fmt: