Clean old temporary local maven index cache dir before scan

[mbien]

When the last local maven repo scan didn't clean up properly, for example when the JVM exits during scan, subsequent scans would throw java.io.IOException: Cannot create temporary directory.

This has a fairly low probability since local index scans finish fast - saw this during debugging.

exception:

java.io.IOException: Cannot create temporary directory: /home/mbien/NetBeansProjects/netbeans/nbbuild/testuserdir/var/cache/tmp-local
	at org.netbeans.modules.maven.indexer.NexusRepositoryIndexerImpl.scan(NexusRepositoryIndexerImpl.java:813)
	at org.netbeans.modules.maven.indexer.NexusRepositoryIndexerImpl.indexLoadedRepo(NexusRepositoryIndexerImpl.java:616)
[catch] at org.netbeans.modules.maven.indexer.NexusRepositoryIndexerImpl.lambda$spawnIndexLoadedRepo$8(NexusRepositoryIndexerImpl.java:714)
	at org.netbeans.modules.openide.util.DefaultMutexImplementation.writeAccess(DefaultMutexImplementation.java:229)
	at org.openide.util.Mutex.writeAccess(Mutex.java:253)
	at org.netbeans.modules.maven.indexer.NexusRepositoryIndexerImpl.lambda$spawnIndexLoadedRepo$9(NexusRepositoryIndexerImpl.java:707)
	at org.openide.util.RequestProcessor$Task.run(RequestProcessor.java:1403)
	at org.netbeans.modules.openide.util.GlobalLookup.execute(GlobalLookup.java:45)
	at org.openide.util.lookup.Lookups.executeWith(Lookups.java:287)
	at org.openide.util.RequestProcessor$Processor.run(RequestProcessor.java:2018)

how to simulate:
create a tmp-local folder in your fresh cache folder, trigger initial local repo indexing, e.g by adding a dependency to a maven project.

[matthiasblaesing]

Only eyeballed this, but I also saw this problem when running test builds (these have the tendency to be forcibly shutdown before scan finished) and approach looks good to me.