Convert from nuget command

the convert-from-nuget command finds all packages.config files and based on them generates dependencies.file + converts packages.config to paket.references. 
If the packages.config is solution-level, then it will be removed, and its deps will be written to dependencies.file
I also added some primitive logic to alter solution file, in order to remove solution-level packages.config.
dependencies.file for the moment will not be added to the solution.

I copied the pattern to find project files (*.*proj).