sub-venv rules: move core dependency to inner make exclusively

[bertsky]

This fixes a problem I've overlooked initially: Having the ocrd (or core) dependency above the MAKELEVEL distinction, i.e. visible both on the inner and outer invocation, is not only unnecessary (as the outer rules strictly only need a bash script), but may break when the outer venv is not fully installed already (which may even happen on an initial checkout when running make -j all). Example:

make ocrd-anybaseocr-crop -W core
. /tmp/ocrd_all/venv/local/sub-venv/headless-tf21/bin/activate && make -C core install PIP_INSTALL="pip3 install "
/bin/sh: 1: .: Can't open /tmp/ocrd_all/venv/local/sub-venv/headless-tf21/bin/activate
Makefile:162: recipe for target '/tmp/ocrd_all/venv/bin/ocrd' failed

Explanation: the outer $(BIN)/ocrd rule gets checked for dependencies in a context where all immediate variables are expanded (make phase 1), with the outer value for VIRTUAL_ENV. So if the outer venv does not exist yet, it gets created. But then that rule's recipe gets executed in a context where all deferred variables are expanded (make phase 2), with the inner value of the sub-venv. But since we are still in the outer make level, we have no rules to create the inner venv yet.

This PR therefore moves all these dependencies into the inner context exclusively.