[MS.Build] Fix metadata item parsing wrong indexing.
[MS.Build] fix support for MSBuildExtensionsPath* to try more than one paths.

So, it seems that where MSBuildExtensionsPath* is evaluated should be likely
evaluated multiple times, and find actually matching files in one of those.
F# build .targets file expects that.
Since it is almost impossible to assume in which usage scenario it is going
to be evaluated multiple times (those properties can be used anywhere by
simple $(...) syntax), it limits the attempts only for Import() so far.
Old xbuild actually does so too. That way, the particular F# issue is fixed.