package , a built-in Tcl command, provides facilities for extending loading new commands into a Tcl interpreter.
DGP, PYK: The man page for pkg_mkIndex covers the basics in the HOW IT WORKS section. For now pkgIndex.tcl is the primary way to make Tcl aware of packages, but stay tuned for better ideas. We don't want to be chained to pkgIndex.tcl files forever.
See also pkg_mkIndex pitfalls.
dgp, Tcl Chatroom, 2014-10-07, made the following comments regarding improvements to package or a similar facility for Tcl: