Support xsl:use-package with xsl:package-location

[Arithmeticus]

Unless I am misreading the specs (which I do commonly enough), there is currently no way for an XSLT writer using xsl:use-package to indicate where the package is to be found, except outside the XSLT environment. I propose to allow xsl:use-package to contain zero or more xsl:package-location children. I propose the addition of an element and not an attribute, because a package may be in multiple locations, and need nuance, as noted below.

Attributes:

• @href, on the model of xsl:import and xsl:include, would specify by relative or absolute URI where the package is.
• @priority (default 0) would provide a mechanism to indicate whether the specified xsl:package-location should override (value greater than 0), or simply provide a fallback for (less than or equal to 0), the preconfigured place the package should be retrieved from.
• @use-when would allow a developer to manage different versions of a package for different cases.

Other attributes given to xsl:package-location would need discussion.