This moves the "legacy"/"nix2" commands under a new `src/legacy/`
directory, instead of being scattered around in a bunch of different
directories.
A new `liblegacy` build target is defined, and the `nix` binary is
linked against it.
Then, `RegisterLegacyCommand` is replaced with `LegacyCommand::add`
calls in functions like `registerNixCollectGarbage()`. These
registration functions are called explicitly in `src/nix/main.cc`.
See: #359
Change-Id: Id450ffc3f793374907599cfcc121863b792aac1a
