The /lib/jim directory contains both dynamically loadable extensions
and pure-Tcl extensions.

Dynamically loadable extensions must have a .so file extension
Tcl extensions must have a .tcl file extension

As long as /lib/jim is in $::auto_path (it is by default), extension
abc can be loaded with:

  package require abc

First abc.so will be tried, and then abc.tcl

Note that this directory may be something like /lib/jim, /usr/lib/jim or
/usr/local/lib/jim, depending upon where jim was installed.