exthelper: add support for "requires" argument for template keyword
This will help use to support the new template API.
# utility to setup pythonpath to point into the tested repositoryexportSRCDIR=`dirname$TESTDIR`if[-n"$PYTHONPATH"];thenexportHGTEST_ORIG_PYTHONPATH=$PYTHONPATHexportPYTHONPATH=$SRCDIR:$PYTHONPATHelseexportPYTHONPATH=$SRCDIRfi