Fix --postgresql-schema to handle quoted schemas

[rbock]

From #106, thanks @MeanSquaredError

The way removal of schemas is currently implemented, it seems to me that it doesn't handle quoted schema names. So even if you specify the correct schema name, if a table name is qualified with a quoted schema name, it will be left intact

[MeanSquaredError]

To summarize the issues that I observed:

• Removing just one of the schema qualifications doesn't make sense. Either none or all of them should be removed simultaneously. Removing just one of them doesn't really make sense.
• Removing the schema qualifications may cause name clashes (duplicate table names) or hide errors in the DDL, e.g.

     CREATE TABLE s1.t (...);
     ALTER TABLE s2.t ALTER COLUMN ... SET DEFAULT...;
  

  With schema qualifications this DDL is invalid, because s2.t is undefined. After the removal of the schema qualification this suddently becomes valid.
• As mentioned earlier, removing a quoted schema qualification doesn't currently work.

[rbock]

If you take a step back and carefully read your first two statements, you will notice that the second is contradicting the first:

If you have a DDL with multiple different schema qualifications, removing all of them (as you suggest) can lead to errors (as you demonstrate).

However, if all tables are schema qualified, removing a single schema qualification (e.g. public.) might make perfect sense.

Anyways, if you feel strongly about this, we should create separate issues for those. This issue is about quoted schemas.

I am also happy to accept PRs, e.g.

• make --postgresql-schema take a list (seems risky for the reasons you explain)
• introduce a special indicator to remove all schema qualifications (ditto).
• rename --postgresql-schema, e.g. --postgresql-drop-schema-qualification.
• make the parameter work with other database backends, too.

But again, those should go with separate issues. Let's please not continue the discussion here.