gnu: python-pre-commit: Move deprecated variable next to replacement.

This fixes a module import dependency cycle, which was introduced in
831baaf81e ("gnu: python-pre-commit: Replace
with pre-commit.").  The rule of thumb to avoid these is that the deprecated
package should live next to the replacement package (in the same module).

* gnu/packages/python-xyz.scm (python-pre-commit): Move to...
* gnu/packages/version-control.scm (python-pre-commit): ... here.

gnu/packages/version-control.scm

@@ -1759,6 +1759,9 @@ specify a list of hooks you want and pre-commit manages the installation and
 execution of any hook written in any language before every commit.")
     (license license:expat)))
 
+(define-public python-pre-commit
+  (deprecated-package "python-pre-commit" pre-commit))
+
 (define-public mercurial
   (package
     (name "mercurial")