The lmod package needs a tclsh. Up until now it just assumed
that one was available on the system.
This change adds a depends_on('tcl') to the lmod package.
The tcl package installs a tclsh script with an embedded version
number (e.g. tclsh8.6) but the lmod configuration looks for tclsh.
This change extends the tcl package to symlink tclshX.Y to tclsh in
the tcl package bin directory.
Package repositories now look like this:
top-level-dir/
repo.yaml
packages/
libelf/
package.py
mpich/
package.py
...
This leaves room at the top level for additional metadata, source,
per-repo configs, indexes, etc., and it makes it easy to see that
something is a spack repo (just look for repo.yaml and packages).
