Creates invalid stylesheet if a phase binds a variable with the same name as a global variable

[dmj]

Given the following Schematron:

<schema xmlns="http://purl.oclc.org/dsdl/schematron" queryBinding="xslt2" defaultPhase="foobar">
  <let name="foobar" value="'...'"/>
  <phase id="foobar">
    <let name="foobar" value="'xxx'"/>
    <active pattern="foobar"/>
  </phase>
  <pattern id="foobar">
    <rule context="*">
      <assert test="false()"/>
    </rule>
  </pattern>
</schema>```

The Skeleton creates an invalid stylesheet. The root cause is the same problem as in #76.