Use Microsoft.IO.Redist in FileUtilities on net472 instead of custom methods

The note in the targets file that sets the variable says these optimizations are relevant for net4.6, we're beyond that so we might be able to use the Microsoft.IO redist ones instead.

1. decouple FEATURE_LEGACY_GETCURRENTDIRECTORY and FEATURE_LEGACY_GETFULLPATH from net472
2. conditionally compile FileUtilities with using Microsoft.IO
3. don't compile the custom methods outside of net35 taskhost project