distro: tcl: Make a `tclsh' symlink.

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

distro: bigloo: Update to 4.0a.

* gnu/packages/scheme.scm (bigloo): Update to 4.0a.

distro: gnutls: Adjust to the latest version.

* gnu/packages/gnutls.scm (gnutls): Remove the #:parallel-build?
  argument, which is no longer needed.  Remove the commented-out LZO
  input, since GnuTLS no longer supports it since 3.0.0.

distro: gnutls: Use the GnuPG mirrors.

* gnu/packages/gnutls.scm (gnutls): Use the gnutls.org/GnuPG URL.

distro: Rename (distro) to (gnu packages).

* distro.scm: Rename to...
* gnu/packages.scm: ... this.  Update all users accordingly.
* Makefile.am (MODULES): Adjust accordingly.
* po/POTFILES.in: Likewise.

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.