Improve documentation on virtual dependencies (#18346)
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## Summary
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The current documentation for virtual dependencies is somewhat confusing
because it places the sentence
> "the package will be built even if a build system is not declared"
immediately after introducing `tool.uv.package = false`. This makes it
sound as if the package might still be built when `package = false` is
set.
In reality, that sentence describes the *default behavior of path
dependencies* (when `package = false` is not set). The revised wording
clarifies that:
- `package = false` → the dependency becomes virtual (its dependencies
are installed, but the package itself is not built or installed)
- otherwise → uv treats the path dependency as a normal package and will
attempt to build it, even without a `[build-system]`.
