Isolate all modules used by luarocks.loader in a luarocks.core.* namespace. Core modules can only require other core modules, and they only use require() at the top-level chunk. In other words, after luarocks.loader is setup, package.path can be altered at will and it should not affect the ability of luarocks.loader to run, and the luarocks.loader modules should not interfere with modules loaded later. This was motivated by @robooo's GSoC work on porting the LuaRocks test suite to Lua using Busted. Busted itself runs using luarocks.loader, and we need to ensure that the modules loaded by the tests do not conflict with the ones used by luarocks.loader. A circular dependency between luarocks.manif and luarocks.search was removed by creating a separate luarocks.manif.writer module. Also, luarocks-admin commands were moved to a luarocks.admin.* namespace.