Git and GitHub support rework

[thegecko]

This pull request fixes the following issues:

• Added support for Git and GitHub hash commits in package specifications
• Fixed the install command to correctly support all non-registry forms
• Fixed the saving of non-registry package specifications to module.json
• Additional tests for the above
• Updated docs for the above

Fixes #801, #691 and #566

@bogdanm @marcuschangarm @yogpan01 @0xc0170 @bremoran @LiyouZhou @korjaa

[autopulated]

🎉 🎊 👍

[bogdanm]

A little late to the party, but :confetti_ball: !