hunter_default_version should be callable from user config

[interj]

Do variables __HUNTER_LAST_DEFAULT_VERSION_NAME, __HUNTER_ALLOW_DEFAULT_VERSION_LOADING and checks related to them protect hunter users against doing anything wrong?
The only effect to these checks I've found is disabling hunter_default_version in user config, which allows you to add a new package without forking whole hunter. You can also change a version of an existing package, but you can do the same with hunter_config.

You can test it by setting this in local config

set(__HUNTER_LAST_DEFAULT_VERSION_NAME "" 
    "Disable alphabetic verification of hunter_default_version calls")
set(__HUNTER_ALLOW_DEFAULT_VERSION_LOADING YES 
    "Allow hunter_default_version calls outside Hunter")
hunter_default_version(some_package_added_to_HUNTER_SELF/cmake/projects VERSION ...)

I've used this for a while and didn't find any problem with it, so maybe those checks should be removed.

[rbsheth]

Hmm, not sure about this. Let's put this on the backlog, for now.