For Tk, also provide a symlink for wishX.X. (#5161)

+ gitk (and possibly other tools) expect to find the Tk executable `wish`, so
  add a symlink with this name that points to the the versioned filename.
+ Example: `wish --> wish8.6`

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

@@ -53,3 +53,8 @@ def setup_environment(self, spack_env, run_env):
     def configure_args(self):
         spec = self.spec
         return ['--with-tcl={0}'.format(spec['tcl'].prefix.lib)]
+
+    @run_after('install')
+    def symlink_wish(self):
+        with working_dir(self.prefix.bin):
+            symlink('wish{0}'.format(self.version.up_to(2)), 'wish')