Build `path` sources without build systems by default (#14413)
We currently treat path sources as virtual if they do not specify a
build system, which is surprising behavior. This PR updates the behavior
to treat path sources as packages unless the path source is explicitly
marked as `package = false` or its own `tool.uv.package` is set to
`false`.
Closes #12015
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Co-authored-by: Zanie Blue <contact@zanie.dev>
