optimize shadow copy from .nuget/packages

[asecchia]

Version Used: any

Steps to Reproduce:

1. open VS
2. open roslyn.sln

We shadow copy analyzers to the temp directory. When the dlls originate in .nuget/packages we can skip the IO cost of copying because the package is immutable.

@mavasani
"We can surely tune our shadow copy based analyzer assembly loader to handle this special case, as long as we are guaranteeing this part for the nuget cache folder: “The reason is it’s supposed to be immutable and we can save the cost of disk copies if we just load it from the original location”. Note that we will lock the file on the disk so user might possibly not even be able to delete analyzer packages until the VS session consuming them is closed."

[mavasani]

Also tagging @srivatsn