Tcl: Fix packages "dde" and "registry" when used from a slave interpreter.

* these packages are statically built-in
* main interpreter has these ready without a "package require" because this
    is done on application initialization
* a "load" within a slave interpreter (invoked due to "package require") of
    one of these DLLs failed because it does not exist
* statically linked packages can be loaded by specifying an empty filename
    --> this is the bugfix