Fix for r72150: rename getSchema() to getSchemaPath(). This terminology creates an...

Fix for r72150: rename getSchema() to getSchemaPath(). This terminology creates an unfortunate and potentially confusing conflict with the PostgreSQL usage of the term. At some point in the future I'd like to have a getSchema() that gives you the PostgreSQL schema name ($wgDBmwschema). Strictly speaking, tables.sql is not a schema, it's an SQL file that creates a schema.