utils: Fix parsing of external path between single quotes

Official subversion documentation only mentions that paths containing spaces must be surrounded by double quotes but we can find some external definitions in the wild whose paths are surrounded by single quotes. Those are properly handled by the official subversion client so we must do the same when parsing externals.

Fix loading of https://svn.code.sf.net/p/delphiasiovst/code reported in SWH-LOADER-SVN-C4