Provide migration tool to move from packages.config project to packagereference

[rrelyea]

Some users would prefer to use VS2017's support for PackageReference, instead of the default Packages.config.

We've provided a way, in tools/options/nuget -- to choose what to use in a new project.
However, many people would like to migrate existing projects as well.

This issue is tracking our future work item to make that migration easier.

Note -- not all packages are compatible with PackageReference, so migration may not work for some projects/packages. See #4942

[gmajian]

There is a GitHubGist for "Converts packages.config into PackageReference at *.csproj project file"
https://gist.github.com/a4099181/074a6c3dd524ea0d343382137492399c

[Kassoul]

An another good tool on Github that use https://github.com/hvanbakel/CsprojToVs2017

[tomkuijsten]

@gmajian that script is very limited, eg doesn't work for packages where it's version number are different then the containing assembly version numbers.

[tomkuijsten]

@Kassoul this tool has the same problem as the script, a lot of references aren't removed.

[tomkuijsten]

@rrelyea do you want to convert to the new csproj format as well?