xsd import from xsd include - xsd

Question

I'm having the following "imp.xsd":
<xsd:schema targetNamespace="http://imported"
xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<xsd:element name="Imported" type="xsd:string"/>
</xsd:schema>
imported from "incl.xsd":
<xsd:schema targetNamespace="http://main"
xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<xsd:import namespace="http://imported" schemaLocation="file:///C:/.../imp.xsd"/>
<xsd:element name="Included" type="xsd:string"/>
</xsd:schema>
which in turn is included from "main.xsd":
<xsd:schema targetNamespace="http://main"
xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<xsd:include schemaLocation="file:///C:/.../incl.xsd"/>
<xsd:complexType name="dummy">
<xsd:sequence xmlns:impt="http://imported" xmlns:incl="http://main">
<xsd:element ref="incl:Included"/>
<xsd:element ref="impt:Imported"/>
</xsd:sequence>
</xsd:complexType>
</xsd:schema>
Question: is "main.xsd" a valid xml schema document?
When validated with the xmllint parser (or from python lxml library, which in turn uses xmllint), I get the following error:
Element '{http://www.w3.org/2001/XMLSchema}element', attribute 'ref':
References from this schema to components in the namespace
'http://imported' are not allowed, since not indicated by an import
statement. WXS schema main.xsd failed to compile
However, another (IBM) parser accepts this XSD without any complaints.
As far as I understand xsd:include, it should behave as if the whole included XSD is inlined into the main document. So, in my opinion, the "main.xsd" should be valid. Is it?
Edit: I was experimenting with xmllint a bit, and when I add the line
<xsd:import namespace="http://imported" schemaLocation="file:///C:/.../imp.xsd"/>
into "main.xsd", xmllint accepts that schema. However, when I add the same line while omitting file:///, I get the following warning:
Element '{http://www.w3.org/2001/XMLSchema}import': Skipping import of
schema located at 'C:/.../imported.xsd' for the namespace
'http://imported', since this namespace was already imported with the
schema located at 'file:///V:/.../imported.xsd'.
Hence I deduce that the original "main.xsd" is valid and that this is a bug in xmllint.

Answer 1

See this thread on the xmlschema-dev#w3.org mailing list. In his answer, Henry S. Thompson quotes following part of the spec:
For a ·QName· to resolve to a schema component [...] the
·namespace name· of the ·QName· is the same as one of the following:
[...]
The ·actual value· of the namespace [attribute] of some <import>
element information item contained in the <schema> element
information item of that schema document [emphasis added].
So indirect imports are not supposed to work.