Migrate more SKIPIF -> EXTENSIONS (#7139)

This is a mix of more automated and manual migration. It should remove all applicable extension_loaded() checks outside of skipif.inc files.

Convert IMAPConnection to IMAP\Connection

IMAP: Disallow direct `new IMAPConnection()` construct

Disallows constructing an `IMAPConnection` class directly with `new IMAPConnection` construct, by throwing an `Error` exception if atte

IMAP: Disallow direct `new IMAPConnection()` construct

Disallows constructing an `IMAPConnection` class directly with `new IMAPConnection` construct, by throwing an `Error` exception if attempted.
`imap_open` is still the only way to create `IMAPConnection` objects.

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