Normalize file names to lowercase ID and version

[joelverhagen]

Changes readers and writers of the global packages folder or V2 style packages folder to make the ID and version component of the path lowercase.

.nupkgs written to package folders have file names {id-lower}.{version-lower}.nupkg, unless we are writting a V2 package folder (e.g. nuget.exe install) with -ExcludeVersion. Then the format is {id-lower}.nupkg.

.nuspec files have file names {id-lower}.nuspec.

Previously, the path to the .nupkg for Newtonsoft.Json 8.0.3 installed locally on Windows would look something like this:

C:\Users\jver\.nuget\packages\Newtonsoft.Json\8.0.3\Newtonsoft.Json.8.0.3.nupkg

After the change, this would look like:

C:\Users\jver\.nuget\packages\newtonsoft.json\8.0.3\newtonsoft.json.8.0.3.nupkg

@emgarten @alpaix @yishaigalatzer @rrelyea

[emgarten]

This looks way better than the previous normalization 😄 Check the xunit functional tests also if you haven't.