Refactor transport and drop git binary dependency

[aymanbagabas]

This PR refactors the transport sub-package and implements server-side git operations like UploadPack and ReceivePack along with delegating client-side operations to SendPack and FetchPack following the canonical git implementation.

• Use shared transport logic across all transports file, git, ssh and http using NeogotiatePack, FetchPack, and SendPack.
• Support HTTP(s) dumb transport (in a follow-up https://github.com/aymanbagabas/go-git/tree/transport-http-dumb)
• Supports git protocol V1
• Support server-side no-progress, sideband, and sideband64k capabilities.
• Add shallow pack support to packfile.Encoder (needed to get the file transport to work with the pure server implementation)
• Fix tests (currently only fails because the above is missing :/)

Future follow-ups:

• Git daemon implementation (git-daemon support)
• HTTP middleware (git-http-backend support)
• go-git standalone cli

Needs: #1082

New transport api: https://gist.github.com/aymanbagabas/1b5ccc265ee62dd574c62d378b4b57bb

Supersedes: #757
Supersedes: #938
Fixes: #883
Fixes: #838