`unused-alias` XSL transformation is too slow on large XMIR files

I'm getting performance warnings when running lints against eo-runtime:

[WARNING] XSL transformation 'unused-alias' took 116ms, whereas threshold is 100ms

What happens: For each alias meta the transformation runs a full document scan:

<xsl:for-each select="/object/metas/meta[head='alias' and count(part)=2]">
  <xsl:if test="count(//o[starts-with(@base, $name)])=0">

With M aliases and N objects this is O(M × N). Programs that import many packages (like eo-runtime) have large M, and the generated XMIR has large N, so the cost multiplies.

What should happen: The transformation completes within the 100ms threshold. Pre-indexing all @base values with xsl:key would reduce each alias lookup to O(1) instead of a full scan.