fix(legacy): import babel once

[hi-ogawa]

Description

import('@babel/core') call is not deduped properly due to await and renderChunk is called in parallel for all chunks. Then weirdly, such duplicated import calls seem to be causing some issues with unbuild stub's interop and I was seeing a following error on main:

$ pnpm -C packages/plugin-legacy/ dev

$ pnpm -C playground/legacy build
...
x Build failed in 467ms
error during build:
[vite:legacy-post-process] babel.transform is not a function
    at Object.renderChunk (/home/hiroshi/code/others/vite/packages/plugin-legacy/src/index.ts:552:28)
    at transformChunk (file:///home/hiroshi/code/others/vite/node_modules/.pnpm/[email protected]/node_modules/rollup/dist/es/shared/node-entry.js:18853:16)
    at file:///home/hiroshi/code/others/vite/node_modules/.pnpm/[email protected]/node_modules/rollup/dist/es/shared/node-entry.js:18927:17
    at async Promise.all (index 8)
    at transformChunksAndGenerateContentHashes (file:///home/hiroshi/code/others/vite/node_modules/.pnpm/rollup@4.28.1/node_modules/rollup/dist/es/shared/node-entry.js:18922:5)
...

I'll try a reproduction on unbuild/jiti later. I couldn't get to the bottom of the issue, but I guess it's okay to have this change regardless. It would be nice if others can reproduce the same issue locally on main.

[hi-ogawa]

I couldn't find a minimal repro. What I'm seeing locally was something like this:

    if (!!babel.transform) {
      console.log("[ok]")
    } else {
      console.log("[not ok]", babel)
    }

$ pnpm -C playground/legacy build
...
[not ok] [Object: null prototype] { default: {} }
[not ok] [Object: null prototype] { default: {} }
[not ok] [Object: null prototype] { default: {} }
[not ok] [Object: null prototype] { default: {} }
[not ok] [Object: null prototype] { default: {} }
[ok]
[ok]
[ok]

I tried JITI_DEBUG=true but still don't have any clue about where this behavior is coming from.