tortoisehg: untie from mercurial and update to 5.0.2 (#66173)

* tortoisehg: untie from mercurial

When I've added `tortoisehg` package, it turned out that mercurial updates
sometimes broke thg. I've added a comment to `mercurial/default.nix` to
update torotisehg version during mercurial updates, but this worked the other way round:
mercurial stopped to be updated regularly (https://github.com/NixOS/nixpkgs/pull/60993).

And sadly it didn't solve the issue - https://github.com/NixOS/nixpkgs/issues/61759.

Here I'm trying to untie them. Tortoisehg will build a custom mercurial package
if versions don't match. This makes updating tortoisehg more complicated,
but it is still better than current situation.

* expose custom mercurial from thg package