[avalanche] Clarify processor dependency to net.h

Summary:
This diff makes the processor dependency to net.h explicit by using forward declaration in netmessagemaker.h, and remove the indirect inclusion of net.h.

Depends on D9851.

Ref T1634.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Maniphest Tasks: T1634

Differential Revision: https://reviews.bitcoinabc.org/D9852