The lmod package should depend_on('tcl')

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.

var/spack/repos/builtin/packages/lmod/package.py

@@ -44,6 +44,7 @@ class Lmod(Package):
     depends_on('lua@5.2:')
     depends_on('lua-luaposix', type=nolink)
     depends_on('lua-luafilesystem', type=nolink)
+    depends_on('tcl')
 
     parallel = False
 

var/spack/repos/builtin/packages/tcl/package.py

@@ -57,3 +57,5 @@ def install(self, spec, prefix):
             configure("--prefix={0}".format(prefix))
             make()
             make("install")
+        with working_dir(prefix.bin):
+            symlink('tclsh{0}'.format(self.version.up_to(2)), 'tclsh')