Add a lockfile mechanism

[savq]

A lot of issues with Paq right now stem from the fact that it doesn't have any kind of persistent state. All information is either in the user's configuration or in the git repo itself. However,

• Expecting the user to add more information in their config defeats the purpose of Paq. They could just use git submodules...
• Querying git using stdio is tedious and error-prone.
  • Lua has very limited string/stream manipulation facilities
  • Using libgit2 bindings seems to be out of the question :(

The easiest way to solve this seems to be storing information in a file so that Paq has an easier time telling what has changed since it was last called.

The lockfile could be used for "reproducible" neovim configs.