Extract general bootclasspath related code into java/bootclasspath.go

The platform_bootclasspath and bootclasspath_fragment modules provide
different capabilities but are related and so have some common
functionality. This change moves some platform_bootclasspath code that
will be of use for bootclasspath_fragment in future into a separate
file.

Bug: 177892522
Test: m nothing
Change-Id: I827b85e33d16155fcc920d553100c9e99267dc4e