Reorganize to go get will work

This makes almost no changes to source, but touches every almost file.

Also fixes error in gremlin test code.
Use Go-conventional filenaming
Don't rename imports unless necessary
Re-add leveldb and fix over-exhuberant .gitignore
Re-add main.go and add install instruction

Updated .gitignore to alloy cayley to be kept.
Move query language packages into query
Reorganise to make cmd code more prominant
Fix non-pathological package shadowing

This is smelly more than anything but I should not have missed it
previously.
Allow assets to come from more places than PWD, do some autodetection, add a build file, and update the README