distro: tcl: Make a `tclsh' symlink.

* gnu/packages/tcl.scm (tcl): Add a `tclsh' symlink.

distro: Change the module name space to (gnu ...).

* distro: Rename to...
* gnu: ... this.  Update module names accordingly.
* Makefile.am: Adjust accordingly.
* po/POTFILES.in: Likewise.
* distro.scm: Search for files under /gnu/packages instead of
  /distro/packages.
* gnu/packages/base.scm (ld-wrapper-boot3): Likewise.

distro: expect: Provide the right path to `stty'.

* distro/packages/tcl.scm (expect): Add #:phases argument.

distro: tcl, expect: Enable tests.

* distro/packages/tcl.scm (tcl, expect): Remove #:tests? argument; add
  #:test-target.

distro: Add Tcl, Expect, and DejaGNU.

* distro/packages/dejagnu.scm, distro/packages/tcl.scm: New files.
* Makefile.am (MODULES): Add them.