Currently pages in gadget namespace must begin with capital (uppercase) letter.

• By convention, JavaScript and CSS files on disk are almost always named with a leading lowercase. It would be good to have parallelism with on-wiki page names (such as while uploading git-managed source files on disk). With the introduction of packaged gadgets (as part of the patch for T198758), it would be preferable to write require('./data.json') rather than require('./Data.json')
• There could be existing pages like MediaWiki:Gadget-Foo.js and MediaWiki:Gadget-foo.js. During the migration, both titles would conflictingly have the same move destination Gadget:Foo.js.

So, I think we should use capitallinkoverride for gadget namespace as well. (It's already being used for gadget definition namespace).