* Make dotnet install tool atomic
Apply TransactionScope to tool install. It can handle the correct timing
of roll back and commit.
Convert existing ToolPackageObtainer and ShellShimMaker by passing logic
via lambda to an object that has IEnlistmentNotification interface. It
turns out the very clean.
Use .stage as staging place to verify of package content, and shim. It
should roll back when something is wrong. When there is ctrl-c, there
will be garbage in .stage folder but not the root of the package folder.
