fetchFromBitBucket: auto-remove an impure file

Also fix the hash in goPackages.inflect, the only user of the fetcher ATM.
Closes #12002 (different `inflect` fix), fixes #12012.
Using fetchzip-derived functions is likely more efficient than fetchhg,
and it's lighter on dependencies (hash is the same as with fetchhg in this case).

pkgs/build-support/fetchzip/default.nix

@@ -10,6 +10,7 @@
 { # Optionally move the contents of the unpacked tree up one level.
   stripRoot ? true
 , url
+, extraPostFetch ? ""
 , ... } @ args:
 
 lib.overrideDerivation (fetchurl ({
@@ -47,7 +48,8 @@ lib.overrideDerivation (fetchurl ({
       fi
     '' else ''
       mv "$unpackDir"/* "$out/"
-    '');
-} // removeAttrs args [ "stripRoot" ]))
+    '') #*/
+    + extraPostFetch;
+} // removeAttrs args [ "stripRoot" "extraPostFetch" ]))
 # Hackety-hack: we actually need unzip hooks, too
 (x: {nativeBuildInputs = x.nativeBuildInputs++ [unzip];})