RFR: 8149843, 8150111 Two small changes to StandardJavaFileManager API

[Jonathan Gibbons]

This is for two small changes to the StandardJavaFileManager API.

JBS: https://bugs.openjdk.java.net/browse/JDK-8149843
StandardJavaFileManager should provide a way to get paths from strings

JBS: https://bugs.openjdk.java.net/browse/JDK-8150111
Need to change signature of StandardJavaFileManager.setLocationFromPaths

With these changes, all standard uses of Paths.get are replaced by using
a user-provided function, defaulting to Paths::get.

The exceptions are:
     file names specified on the javac command line
     file names used in some internal debugging/tracing features in javac
     file names used in sjavac

Combined specdiff: http://cr.openjdk.java.net/~jjg/8149843/specdiff.00/
Combined webrev: http://cr.openjdk.java.net/~jjg/8149843/webrev.00/

-- Jon