Revert "treewide: replace rev with tag"

This reverts commit 65a333600d.

This wasn't tested for correctness with something like fodwatch [0],
and should not have been (self-)merged so quickly, especially without
further review.

It also resulted in the breakage of at least one package [1] (and that's
the one we know of and was caught).

A few packages that were updated in between this commit and this revert
were not reverted back to using `rev`, but other than that, this is a
1:1 revert.

[0]: https://codeberg.org/raphaelr/fodwatch
[1]: https://github.com/NixOS/nixpkgs/pull/396904 / 758551e458

pkgs/development/python2-modules/pip/default.nix

@@ -18,7 +18,7 @@ buildPythonPackage rec {
   src = fetchFromGitHub {
     owner = "pypa";
     repo = pname;
-    tag = version;
+    rev = version;
     sha256 = "0hkhs9yc1cjdj1gn9wkycd3sy65c05q8k8rhqgsm5jbpksfssiwn";
     name = "${pname}-${version}-source";
   };

pkgs/development/python2-modules/pycairo/default.nix

@@ -20,7 +20,7 @@ buildPythonPackage rec {
   src = fetchFromGitHub {
     owner = "pygobject";
     repo = "pycairo";
-    tag = "v${version}";
+    rev = "v${version}";
     sha256 = "142145a2whvlk92jijrbf3i2bqrzmspwpysj0bfypw0krzi0aa6j";
   };
 

pkgs/development/python2-modules/setuptools/default.nix

@@ -20,7 +20,7 @@ let
     src = fetchFromGitHub {
       owner = "pypa";
       repo = pname;
-      tag = "v${version}";
+      rev = "v${version}";
       sha256 = "0z3q0qinyp1rmnxkw3y5f6nbsxhqlfq5k7skfrqa6ymb3zr009y1";
       name = "${pname}-${version}-source";
     };

pkgs/development/python2-modules/wheel/default.nix

@@ -14,7 +14,7 @@ buildPythonPackage rec {
   src = fetchFromGitHub {
     owner = "pypa";
     repo = pname;
-    tag = version;
+    rev = version;
     sha256 = "sha256-JlTmUPY3yo/uROyd3nW1dJa23zbLhgQTwcmqZkPOrHs=";
     name = "${pname}-${version}-source";
     postFetch = ''