Remove usage of default directory for mixed refs builds

[sharon-wang]

In a regular build, sources are generated into the default directory for non-compressed refs build and the compressedrefs directory for a compressed refs build.

In mixed builds, we currently generate the sources/libs into the default directory. However, it's not actually necessary for a directory to be used at all for a mixed build, since sources/libraries don't need to be kept separate. So, the generated libraries can just sit in the parent directory where the compressedrefs/default directories currently sit alongside all the other libraries, i.e. images/jdk/lib, jdk/lib.

More mixed builds discussion: #8878.

[gacholio]

I'm not sure we want to do this - we had originally decided that default would be the directory for mixed builds.

[sharon-wang]

Right, using default keeps the JVM libraries contained and separated from the rest of the libraries. Is this separation something we want to keep going forward? If so, I can close this issue.

I think @keithc-ca might have some thoughts on this too?

[gacholio]

The plan was to allow the redirector to continue to select other VMs if we have a future experimental build that we want to package in. This doesn't necessarily preclude moving the default VM up a level, but I'm not sure there's any real value to it.

[gacholio]

@tajila

[sharon-wang]

If no other comments, I think I'll just close this issue.

[keithc-ca]

It might make a (small?) performance difference removing an extra directory from each of these search paths:

com.ibm.oti.vm.bootstrap.library.path
java.library.path
sun.boot.library.path

It takes longer to search a directory and fail than it does to succeed.

We don't need to eliminate the redirector, so those experiments would still be possible.