Removed provider@user/repo support.

1. Removed provider@user/repo support.
2. Added '_' to allow character for url  regex.

Signed-off-by: Rack Lin <racklin@gmail.com>
In my orignal idea that provider was singleton class and only created at getProviders / getProviderByName. 

And a new class object `Package?` was been created and return from `provider->exists` . 
`Package` object is data object , that only contains `name` , `version`, `url` .

But i don't spent too much times to implements that.

Maybe refactoring next time., refactoring was never ending..