fix(rolldown): keep filtered-out importers from hiding modules in the module graph

[IWANABETHATGUY]

The Rolldown module graph picked its roots with importers.length === 0. That checks importers across the whole build, but the graph only gets the modules left after the file-type and package filters. A module whose importers were all filtered out was neither a root nor anyone's child, so it disappeared. When no remaining module had zero importers, the graph was empty.

Roots are now the modules with no importer left in the filtered list, falling back to all modules when every one has an importer. This is the rule the Vite module graph already uses (packages/vite/src/app/components/modules/Graph.vue). The unfiltered graph is unchanged.

Screenshots

These screenshots show Rolldown DevTools inspecting its own frontend: the client build of the Nuxt app in packages/rolldown (entry nuxt/dist/app/entry.js). pnpm build in this repo builds it with Rolldown and records the session in packages/rolldown/node_modules/.rolldown/. "Before" is main at 89a7090, "after" is this PR.

To reproduce: run pnpm build, then pnpm -C packages/rolldown run dev, open the session for nuxt/dist/app/entry.js, switch Modules to Graph, and apply the filter. The dev server currently rejects the client with "Not authorized by the devframe server" despite clientAuth: false in its Nuxt config, so I started it with VITE_DEVTOOLS_DISABLE_CLIENT_AUTH=true.

CSS filter: empty graph before, 4 CSS modules after.

Package filter (floating-vue): empty graph before, both modules after.

Vue filter: before, app.vue and the modules reachable only through it are missing. The lazy-loaded pages still show because they have no static importer. After, app.vue is a root.

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commit: b78ee3c