replay: Fix external tuple use in process_external method

Some required changes were missed in 20c1445f, fortunately that commit is not yet tagged neither deployed.

The external tuple created at the beginning of the process_external method was missing a third boolean member.

The externals defintions in DirState was also missing that boolean member.

Test checking if an external was already exported was invalid.

Also alias the type of that tuple for better code readability.

Related to #611

Depends on !123 (closed)

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Migrated from D7128 (view on Phabricator)