Replace use of autosetup/lib/*.auto for sqlite-custom-... features to avoid the possi...

Replace use of autosetup/lib/*.auto for sqlite-custom-... features to avoid the possibility of multiple files being loaded (in an unpredictable order) for that purpose. Instead look for autosetup/sqlite-custom.tcl and source it if it exists. The intent is that sqlite-custom.tcl only ever be added in vendor-specific branches and never in the trunk.

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