[3.9] Simplify Import XSL and allow usage of external files

[BartChris]

Adresses #6730

The XSL logic for the import uses more complex structures then the one for the export.

Export:

kitodo-production/Kitodo/src/main/java/org/kitodo/export/XsltHelper.java

Line 48 in 3e57d4f

static ByteArrayOutputStream transformXmlByXslt(StreamSource source, URI xslFile)

Import:

kitodo-production/Kitodo-XML-SchemaConverter/src/main/java/org/kitodo/xmlschemaconverter/XMLSchemaConverter.java

Line 105 in 3e57d4f

private String transformXmlByXslt(String xmlString, InputStream stylesheetFile) {

It also fails in resolving external files during the XSL based import. (See #6730). I tried to simplify the import XSL logic by making it more similar to the export logic. Have not stumbled over problems so far. External files are now also supported during import XSL transformations.

[BartChris]

I tested with this simple mapping file:

mapping.xml

<mappings>
    <map from="#test" to="external mapping works"/>
</mappings>

xsl-file:

<xsl:variable name="mapping-root" select="document('mapping.xml')/mappings"/>

<xsl:variable name="testMap" select="$mapping-root/map[@from='#test']/@to"/>
<kitodo:metadata name="MappingTest">
   <xsl:value-of select="if ($testMap) then $testMap else 'mapping.xml not loaded'"/>
</kitodo:metadata>