Allow users to set `package = true` on `tool.uv.sources` (#12014)
## Summary
In https://github.com/astral-sh/uv/issues/11998, a user is attempting to
vendor `pydantic-core`. But when they add `pydantic-core = { path =
"src/foo/vendor/pydantic-core" } `, we're installing it as a virtual
package, since `pydantic-core/pyproject.toml` contains `package =
false`.
This PR allows users to mark dependencies as "explicitly a package" or
"explicitly not a package" (i.e., virtual), as a workaround.
Closes https://github.com/astral-sh/uv/issues/11998.
