Enable installing with go get

[jlegrone]

What this PR does / why we need it:

This moves main.go to a directory that matches the name of our installed binary, as recommended by the cobra docs:
https://github.com/spf13/cobra/blob/7547e83b2d85fd1893c7d76916f67689d761fecb/README.md#getting-started

This will make it possible to install ct with the command go get github.com/helm/chart-testing/ct if merged to master.

Which issue this PR fixes

Closes #109

Special notes for your reviewer:

Since go get doesn't allow specifying a branch, for testing purposes I modified imports on the master branch of my fork. You can run go get github.com/jlegrone/chart-testing/ct to test go get behavior.

[rimusz]

nice one