In section 3.13.1 Conditional Element Inclusion we mention the following about its use on xsl:stylesheet and xsl:transform:

"If the xsl:stylesheet or xsl:transform element itself is effectively excluded, the effect is to exclude all the children of the xsl:stylesheet or xsl:transform element, but not the xsl:stylesheet or xsl:transform element or its attributes."

It seems to make sense to include xsl:package here as well. I don't necessarily see a use-case for it (but I don't see one for xsl:stylesheet/xsl:transform either), but neither do I see how otherwise a package is supposed to behave if use-when="false()" on xsl:package.