Resolves a system identifier (fragmentless URI reference) or a
public identifier into a new input source. It is typically used
when a URI reference is encountered in the original stream and
needs to be resolved (e.g. to support an external entity
reference, XInclude, xsl:include/import, document(), etc.).
The hint parameter is used to give a hint as to what the
resolution will be used for.
The default implementation ignores the hint, and will resolve
against a catalog, if one is available.
If the ignoreErrors flag is set, an error during resolution
(such as "file not found") will result in None's being returned,
rather than a raised exception.