Class names case conflicts

[tefra]

Currently the generators tries to create two class A and two class B, thankfully the class resolver raises an exception before that.

    raise ResolverValueError(f"Duplicate class: `{obj.name}`")
E   xsdata.exceptions.ResolverValueError: Duplicate class: `A`

<?xml version="1.0" encoding="utf-8" ?> 
<xs:schema id="repro" 
                  elementFormDefault="qualified"
                  xmlns:xs="http://www.w3.org/2001/XMLSchema">

 	<xs:element name="A" type="base-A" substitutionGroup="item"/>
 	<xs:element name="B" type="base-B" substitutionGroup="item"/>
 	<xs:element name="a" type="A" substitutionGroup="item"/>
 	<xs:element name="b" type="B" substitutionGroup="item"/>