nix-prefetch-url: Support prefetching from a Nix expression

For example,

  $ nix-prefetch-url -A hello.src

will prefetch the file specified by the fetchurl call in the attribute
‘hello.src’ from the Nix expression in the current directory. This
differs from ‘nix-build -A hello.src’ in that it doesn't verify the
hash.

You can also specify a path to the Nix expression:

  $ nix-prefetch-url ~/Dev/nixpkgs -A hello.src

List elements (typically used in ‘patches’ attributes) also work:

  $ nix-prefetch-url -A portmidi.patches.0

doc/manual/command-ref/nix-prefetch-url.xml

@@ -20,6 +20,7 @@
   <cmdsynopsis>
     <command>nix-prefetch-url</command>
     <arg><option>--type</option> <replaceable>hashAlgo</replaceable></arg>
+    <arg><option>--print-path</option></arg>
     <arg choice='plain'><replaceable>url</replaceable></arg>
     <arg><replaceable>hash</replaceable></arg>
   </cmdsynopsis>