Automatically use package.json's dependencies for rollupOptions.external when in lib mode

[Shinigami92]

I switched some of my projects from using tsc to build with vite so I can benefit from a nice CLI output and bundling cjs, es(m) and dts

While doing that I found out that I need to configure build > rollupOptions > external to contain the package.json's dependencies so they don't get bundled inside the dist's content.
IMO this should be the default behavior so we don't need to explicitly configure that.

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Changing this would result in a breaking change, so if we want that nice-to-have, it must be published in a new major version.
I can think of that if build > lib > entry is set to a value and build > rollupOptions > external is undefined, then use dependencies array as default value for external.
We will still have the possibility to change the behavior in our configs by overriding that default behavior by just configure external on demand.

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Some things I found while searching through already existing issues / discussions in Vite Repo:

• Cross-Link: Library mode: How should I manage external dependencies? #6198
• There is a rollup-plugin-node-externals, but I don't want to install a plugin, because the goal is to reduce the config for default behavior.

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Off-Topic extra thought: Might it be possible to extract library-mode completely out of Vite's Core and just provide a plugin? 👀
vitest already somehow changed and opt-in to Vite's defineConfig to provide the option property test. Maybe we can add via the same way support for build > lib 🤔

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Edit: started with an implementation

• feat!: auto resolve deps in lib-mode #10632

[haoqunjiang]

In that case, I think we need to phase out UMD support in library mode by default too.
Because we can't automatically infer the external global variable names for the UMD build.

It's not supported when using multiple entries anyway #10315