[modules] Polluted manifest in successive build:manifest calls

[Timeraa]

Describe the bug

Just noticed that in dev mode if you change a file like the background.ts in the demo extension the build:manifest hook of the new auto-icons module sends the icons property found console log which is due to the manifest.icons being filled out by the module the first dev run but the build:manifestGenerated hook should always get the "clean" manifest only the generated steps at that point in time

Reproduction

cd packages/wxt-demo
pnpm dev
# Save entrypoints/background.ts and you'll see the auto-icon warnings

Steps to reproduce

1. Run dev in the demo extension
2. Notice no "icons property found" log
3. Update background.ts
4. Notice "icons property found" log

System Info

System:
    OS: macOS 14.5
    CPU: (8) arm64 Apple M1 Pro
    Memory: 217.91 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 20.15.1 - /usr/local/bin/node
    npm: 10.7.0 - /usr/local/bin/npm
    pnpm: 8.15.9 - /usr/local/bin/pnpm
    bun: 1.1.1 - ~/.bun/bin/bun
  Browsers:
    Safari: 17.5
  npmPackages:
    wxt: workspace:* => 0.19.0

Used Package Manager

pnpm

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