`spack buildcache list` did not have a way to display the namespace of
packages in the buildcache. This PR adds that functionality.
For consistency's sake, it moves the `-N/--namespace` arg definition to
the `common/arguments.py` and modifies `find`, `solve`, `spec` to use
the common definition.
Previously, `find` was using `--namespace` (singular) to control whether
to display the namespace (it doesn't restrict the search to that
namespace). The other commands were using `--namespaces` (plural). For
backwards compatibility and for consistency with `--deps`, `--tags`,
etc, the plural `--namespaces` was chosen. The argument parser ensures
that `find --namespace` will continue to behave as before.
Co-authored-by: Harmen Stoppels <me@harmenstoppels.nl>
a14f4b5a02