Improve info variant header (#14275)

In "spack info" the Variants header currently has two blank
lines under it. That's too much. It looks like the actual
content belongs to something else.

Instead underline the headers to make things more obvious.

lib/spack/spack/cmd/info.py

@@ -106,7 +106,9 @@ def lines(self):
             yield '    None'
         else:
             yield '    ' + self.fmt % self.headers
-            yield '\n'
+            underline = tuple([l * "=" for l in self.column_widths])
+            yield '    ' + self.fmt % underline
+            yield ''
             for k, v in sorted(self.variants.items()):
                 name = textwrap.wrap(
                     '{0} [{1}]'.format(k, self.default(v)),