tcl8.6-dev is definitely needed. Many distros these days split software packages up into "files you need to run the software" and "files you need to compile & link code against the software." tclConfig.sh falls in the latter category - tcl works just fine without it until you want to compile something against it. There's no reason you should need to modify PATH or uninstall the tcl8.6 base package though (tcl8.6-dev depends on it so apt would have reinstalled it straight away).