Do not attempt prefixing cross database / schema queries.

Problem/Motivation

You have a Drupal 6 database and a Drupal 8 database. Migration puts the id mapping table in the Drupal 8 database because that's the only database it knows it has write permissions to. It tries to join this table against tables in the Drupal 6 database. This breaks if a default prefix is set because Select::__toString unconditionally prefixes every table name and so it'll try do defaultprefix_dbname.tablename. On MySQL this foo.bar notation would mean foo is a database, on PostgreSQL it would it's a schema.

Note that the semi-common practice of using prefixes with dots have absolutely no bearing on this patch. It's the actual table names across databases / schemas table names containing dots are not supported.

Proposed resolution

Do not prefix table names if they contain a dot. Trust the caller to prefix the table name as necessary for that database. Do not trust it to escape however. Never trust the caller with security. And this is fine: escapeTable is already keeping the dots. Note this is reasonably backwards compatible because previously such joins simply didn't work as the prefix would be applied to the database name. It is not impossible that someone actually used default or per table prefixes to make such joins do something tricky but for my life I can't imagine such a scenario. (And I have a very wild imagination in these matters. Also called experience.)

A test would only be possible if there were phpunit tests for the database as without mocking you'd need two databases to test this against. So I don't think a test is feasible to be required at this point.