Project import should not use JVM exit hooks to cleanup directories

The project import process implemented by AbstractContinuumProjectBuilder.createMetadataFile(...) adds a JVM hook to schedule removal of generated project directories. This suffers from at least the following problems:

• files grow unbounded, since they will not be removed until JVM shutdown
• if the JVM does not shutdown cleanly, the files are never removed
• the number of JVM hooks keeps growing as projects are added

Rather than deferring until shutdown, project files should be removed immediately after the projects are imported.