fix(lib): respect rollupOptions.input in lib mode

[lsdsjy]

Fixes #9086

Description

#7275 introduced a regression which caused rollupOptions.input to be ignored in library mode. Before that, the de facto behaviour in library mode is rollupOptions.input overwriting lib.entry. This PR tweaks the config merging behaviour so that rollupOptions.input is only ignored in SSR mode.

Additional context

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[DrJume]

This PR breaks the cypress-vite preprocessor:

• vite@3.1.3 breaks cypress-vite mammadataei/cypress-vite#5

Error: Invalid value for option "output.inlineDynamicImports" - multiple inputs are not supported when "output.inlineDynamicImports" is true.
    at error (file:///runner/_work/<project>/node_modules/.pnpm/rollup@2.78.1/node_modules/rollup/dist/es/shared/rollup.js:1858:30)
    at getInlineDynamicImports (file:///runner/_work/<project>/node_modules/.pnpm/rollup@2.78.1/node_modules/rollup/dist/es/shared/rollup.js:23490:16)
    at normalizeOutputOptions (file:///runner/_work/<project>/node_modules/.pnpm/rollup@2.78.1/node_modules/rollup/dist/es/shared/rollup.js:23392:34)
    at getOutputOptions (file:///runner/_work/<project>/node_modules/.pnpm/rollup@2.78.1/node_modules/rollup/dist/es/shared/rollup.js:23781:12)
    at getOutputOptionsAndPluginDriver (file:///runner/_work/<project>/node_modules/.pnpm/rollup@2.78.1/node_modules/rollup/dist/es/shared/rollup.js:23776:12)
    at handleGenerateWrite (file:///runner/_work/<project>/node_modules/.pnpm/rollup@2.78.1/node_modules/rollup/dist/es/shared/rollup.js:23751:74)
    at Object.write (file:///runner/_work/<project>/node_modules/.pnpm/rollup@2.78.1/node_modules/rollup/dist/es/shared/rollup.js:23721:20)
    at generate (file:///runner/_work/<project>/node_modules/.pnpm/vite@3.1.8/node_modules/vite/dist/node/chunks/dep-4da11a5e.js:45818:64)
    at doBuild (file:///runner/_work/<project>/node_modules/.pnpm/vite@3.1.8/node_modules/vite/dist/node/chunks/dep-4da11a5e.js:45826:32)
    at async Module.build (file:///runner/_work/<project>/node_modules/.pnpm/vite@3.1.8/node_modules/vite/dist/node/chunks/dep-4da11a5e.js:45662:16)
    at async Object.handler (/runner/_work/<project>/node_modules/.pnpm/cypress-vite@1.2.0_vite@3.1.8/node_modules/cypress-vite/dist/index.js:[97](https://github.com/falcondev-it/falcon-cookie/actions/runs/3275806652/jobs/5391101940#step:12:98):21)

[patak-dev]

@DrJume would you create a new issue so we can properly track the regression?