Swiftmailer import should not be an EmailBuilder

Problem/Motivation

The class SwiftmailerEmailBuilder exists as a convenient way to carry the config import. However it's not so great:

• It creates an extra step in the migration from Swiftmailer.
• It generates the possibility for creating meaningless Mailer Policy for Swiftmailer.
• It will work even less well after #3315358: Allow full management of override plugins because enabling/disabling of it won't make sense.

Proposed resolution

Remove SwiftmailerEmailBuilder, instead import policy automatically in symfony_mailer_install().

Remaining tasks

User interface changes

API changes

Data model changes