Fixing the missing port issue with Nessus

[quentinpraz]

As discussed in issue #171, when importing targets from a Nessus report, only one port per IP/Host was actually scanned. This limitation occurred because the target list was implemented as a map[string]int.
To address this issue, I made the following changes:

• Modified the target list to use a map[string][]int instead, allowing multiple ports per IP/Host.
• Refactored the functions to accommodate the updated target list.
• Additionally, I introduced a function to remove any duplicate ports for the same IP/Host, ensuring each port is scanned only once.

[leonjza]

Thanks! 🥳