Pages created by "stateful" plugins can get deleted (e.g. those in src/pages)

[KyleAMathews]

Something caused (exposed?) by #11831

[KyleAMathews]

gatsby/packages/gatsby/src/bootstrap/page-hot-reloader.js

Lines 61 to 70 in 057f1ce

	// Delete pages that weren't updated when running createPages.
	Array.from(store.getState().pages.values()).forEach(page => {
	if (
	!_.includes(statefulPlugins, page.pluginCreatorId) &&
	page.updatedAt < timestamp
	) {
	deleteComponentsDependencies([page.path])
	deletePage(page)
	}
	})