with_namespace, local_namespace: Propagate warn.conflicts

[kyleam]

When with_namespace() and local_namespace() gained a warn.conflicts
parameter in 4636e74 (Do not warn about conflicts by default,
2017-11-15), the underlying attach() calls were updated to pass a
hard-coded warn.conflicts = FALSE. This was presumably
unintentional because 1) there's no comment or documentation about why
these functions expose warn.conflicts while ignoring it and 2) all
other functions in namespace.R propagate this argument.

Make with_namespace() and local_namespace() pass along the
warn.conflicts argument.

[kyleam]

All of the ci failures look like unrelated errors in the setup-r-dependencies step.

[kyleam]

All of the ci failures look like unrelated errors in the setup-r-dependencies step.

I've rebased to resolve conflicts in NEWS.md, and the latest ci run is all green.

[lionel-]

Thanks!