sparse zones now explicitly add /usr packages already in the global zone

[mounty1]

I think the title reflects the problem.

In r151042 and before, it was possible to create a sparse zone and install, for example, just the cyrus-mail package in it. Although that package depends on the libxslt package, it was enough that libxslt was installed in the global zone, since the sparse zone shares /usr and thus can use the libxslt contents such as usr/bin/amd64/xslt-config.
In r151044 this no longer works:

# pkg install -R /zone/mail/root cyrus-mail
Creating Plan (Evaluating mediators): /
pkg install: The files listed below match exclusions which are configured
on this image and can therefore not be installed:

usr/bin/amd64/xslt-config
usr/bin/amd64/xsltproc
usr/bin/i386
[...]

[stale]

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