Add source to detailed logging (consider for 3.5)

[MarkOsborneMS]

Microsoft's internal CoreXT tool has been running with a fork of nuget that adds additional logging indicating the source when a package is download from a remote source. e.g.:
Added package 'Newtonsoft.Json.9.0.1' to folder 'E:\repos\mytest\packages' from source 'https://api.nuget.org/v3/index.json'

Adding this functionality to nuget would allow CoreXT to get back onto the mainline nuget rather than maintain its own fork.

@rrelyea please consider for 3.5
/cc @yishaigalatzer @harikmenon

See PR: NuGet/NuGet.Client#783

[joelverhagen]

Merged into dev with NuGet/NuGet.Client@b335ef9. @rrelyea, we spoke offline about this potentially going into 3.5.