Reorg to make go install work

[donatj]

This is a small change, but it allows standard go install to work as expected.

$ go install github.com/dnaeon/makefile-graph/cmd/makefile-graph@latest

See: https://github.com/golang-standards/project-layout/tree/master/cmd

[codecov-commenter]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered ☂️

[dnaeon]

Hey @donatj ,

Thanks for the PR! Could you also please update the README and add the details about go install ... in it?

This part of the README can be replaced with your example of go install ...

Install the CLI application using go build.

go build -o $(go env GOBIN)/makefile-graph github.com/dnaeon/makefile-graph/cmd

Thanks again!

[donatj]

Sure thing! I have updated the README.

[dnaeon]

Thanks!