Replace SourcePath with VFS

[fwcd]

The IntelliJ SDK provides a virtual file system (documented here) that could replace the current source path implementation and would integrate better with #190.

Implementation notes:

• Virtual files have an associated charset and can be manipulated dynamically using OutputStreamWriters.
• The global VFS can be fetched from VirtualFileManager.getInstance()
• Parsed KtFiles should be managed by the KotlinCoreEnvironment
• Updating a build-related file should result in a call to KotlinCoreEnvironment.updateClasspath instead of replacing the entire Compiler instance (as is currently done in CompilerClassPath)
• SourcePath should be a shallow abstraction over the VFS or removed entirely