Print positions in --debugger, instead of pointers

[9999years]

The --debugger currently prints the pointers of source code positions, rather than the positions themselves.

Before:

0: while evaluating the attribute 'python311.pythonForBuild.pkgs'
0x600001522598

After:

0: while evaluating the attribute 'python311.pythonForBuild.pkgs'
/nix/store/hg65h51xnp74ikahns9hyf3py5mlbbqq-source/overrides/default.nix:132:27

   131|
   132|       bootstrappingBase = pkgs.${self.python.pythonAttr}.pythonForBuild.pkgs;
      |                           ^
   133|     in

[9999years]

TODO: Needs a changelog entry.