Allow dashes in identifiers

In Nixpkgs, the attribute in all-packages.nix corresponding to a
package is usually equal to the package name.  However, this doesn't
work if the package contains a dash, which is fairly common.  The
convention is to replace the dash with an underscore (e.g. "dbus-lib"
becomes "dbus_glib"), but that's annoying.  So now dashes are valid in
variable / attribute names, allowing you to write:

  dbus-glib = callPackage ../development/libraries/dbus-glib { };

and

  buildInputs = [ dbus-glib ];

Since we don't have a negation or subtraction operation in Nix, this
is unambiguous.

doc/manual/release-notes.xml

@@ -14,6 +14,11 @@
 
 <itemizedlist>
 
+  <listitem>
+    <para>Dashes are now valid as part of identifiers and attribute
+    names.</para>
+  </listitem>
+
   <listitem>
     <para>Nix no longer sets the immutable bit on files in the Nix
     store.  Instead, the recommended way to guard the Nix store