[rubygems/rubygems] Refactor `set_paths!`

The method has two different concerns not straightforward to
distinguish.

One is to set a new `install_path`, which is done by setting the
`@install_path` instance variable, and invalidating the `local_specs`
memoization, so that the next time they are picked from the new
location.

The other one is to set a new `cache_path` which is done by setting the
`@cache_path` instance variable, and invalidating the `git_proxy`
memoization, so that the next git proxy uses the new cache location.

This commit splits the logic so that this is easier to understand.

https://github.com/rubygems/rubygems/commit/55904094e8

lib/bundler/source/git.rb

@@ -302,8 +302,18 @@ module Bundler
       end
 
       def set_paths!(path)
-        @local_specs = @git_proxy = nil
-        @cache_path  = @install_path = path
+        set_cache_path!(path)
+        set_install_path!(path)
+      end
+
+      def set_cache_path!(path)
+        @git_proxy = nil
+        @cache_path = path
+      end
+
+      def set_install_path!(path)
+        @local_specs = nil
+        @install_path = path
       end
 
       def has_app_cache?