Preparation: Flatten src and make directories into one

With the repository consolidated (JDK-8165183), it makes organization sense to move the contents of
    corba/src
    jaxp/src
    jaxws/src
    jdk/src
    langtools/src
    nashorn/src
into
    $OPENJDK/src

For example, to move
    /jdk/src/java.base
to
    /src/java.base
and similarly for each directory corresponding to a module.
    For the HotSpot sources, it is less clear to me what to do. Perhaps the two module directories in HotSpot jdk.hotspot.agent and jdk.vm.ci should be moved into $OPENJDK/src with the remaining files moved into $OPENJDK/src/hotspot/{cpu, os, os_cpu, share}.

The {$OLD_REPO}/make contents should also be consolidated in some sensible way into a single /make directory.