Search for tools in NuGet package folders.

[aarondandy]

The BauNuGet plugin currently needs the ability to locate the nuget.exe executable within the packages folder. Recently while working with FluentMigrator I also needed to locate another executable within a package folder. This is beginning to become a thing, maybe later it could extend to stylecop or other popular tools.

Some possible signatures:

SomethingSomething.FindInPackage(packageName: "FluentMigrator", fileName: "migrate.exe");

SomethingSomething.FindInPackage(packageName: "FluentMigrator", fileName: "migrate.exe", internalPath: "tools/IA64");

This functionality could replace item 2 in https://github.com/bau-build/bau-nuget/issues/28