https://www.isotc211.org/2005/gfc/gfc.xsd imports non-existent xsds

[srstsavage]

https://www.isotc211.org/2005/gfc/gfc.xsd still tries to import XSDs from http://standards.iso.org/ittf/PubliclyAvailableStandards/ISO_19139_Schemas, which no longer exists.

<xs:import namespace="http://www.isotc211.org/2005/gmd" schemaLocation="http://standards.iso.org/ittf/PubliclyAvailableStandards/ISO_19139_Schemas/gmd/citation.xsd"/>
<xs:import namespace="http://www.isotc211.org/2005/gco" schemaLocation="http://standards.iso.org/ittf/PubliclyAvailableStandards/ISO_19139_Schemas/gco/gco.xsd"/>
<xs:import namespace="http://www.isotc211.org/2005/gmx" schemaLocation="http://standards.iso.org/ittf/PubliclyAvailableStandards/ISO_19139_Schemas/gmx/gmx.xsd"/>

These should be updated to their schemas.isotc211.org equivalents

<xs:import namespace="http://www.isotc211.org/2005/gmd" schemaLocation="https://schemas.isotc211.org/schemas/19139/-/gmd/1.0/citation.xsd"/>
<xs:import namespace="http://www.isotc211.org/2005/gco" schemaLocation="https://schemas.isotc211.org/schemas/19139/-/gco/1.0/gco.xsd"/>
<xs:import namespace="http://www.isotc211.org/2005/gmx" schemaLocation="https://schemas.isotc211.org/schemas/19139/-/gmx/1.0/gmx.xsd"/>

(originally reported here)