Fix base_dir updating for nested .xsd dependencies

[deelin]

Say we have the directory structure as follows:

/BASE.xsd --> Depends on deps/Dep1.xsd
/deps/Dep1.xsd --> Depends on Dep2.xsd
/deps/Dep2.xsd

Parsing BASE.xsd from within its containing folder would result in base_dir being . for all nested folders within. So when ./deps/Dep1.xsd is parsed, it will be unable to find Dep2.xsd because it will look in the parent directory . instead of ./deps

[plq]

I was not aware of this behaviour. Can you please also write a test for it?

[plq]

BTW, thanks a lot for the patch!