[xbuild] Fix typo that broke 2.0/3.5 targets
[xbuild] Make ToolLocationHelper work better for 2.0/3.5

Mono keeps unversioned tools in the 4.5 directory, so look there
for 2.0. Additionally, most 3.5 tools are in the 2.0 directory.
[xbuild] Fix Tooltask.ToolPath behaviour to match .NET