(maint) Unset the defaultprovider after caching it

Prior to this change the defaultprovider for package was implicitly cached
in both package_spec.rb and msi_spec.rb (just by checking it). This represented
spec "leakage" which could result in downstream spec failures on subsequent
tests which accessed defaultprovider. E.g. this sequence failed:

be rspec spec/unit/provider/package/msi_spec.rb spec/unit/type/package_spec.rb

This change clears the defaultprovider for package after caching it for
both these two spec tests.

However, there are a lot of uses of defaultprovider in spec tests, so we may
want a more general solution.