TCLLIBPATH was not actually extended if a package's path was a substring
of a package already on the path. This commit fixes that behavior.
Additionally, '--prefix' is now used in place of '--set', as the latter
was found to break tkremind from the remind package.
Implement tclPackageHook, a setup hook that adjusts TCLLIBPATH to include the
paths of any installed Tcl packages, propagates that TCLLIBPATH to
anti-dependencies, and wraps any installed binaries to set their TCLLIBPATH.
Additionally, implement a makePythonPackage-style mkDerivation wrapper to use
reasonable defaults for Tcl packages and use tclPackageHook.
