[install] sanitise against malformed bun.lockb

[alexlamsl]

fixes #2392

[Jarred-Sumner]

The unique packages bitset was useful for checking if the same package already exists

I’m confused why it’s unnecessary

[alexlamsl]

AFAICT unique_packages has not been in use since the deduplication code got commented out − historically, it was used to populate duplicate_resolutions_bitset which in turn was not being read anywhere else either 🤔

[Jarred-Sumner]

Thank you