No lib in project included: ic2.core.utils.collection.CollectionUtils
error: package ic2.core.utils.collection does not exist
import ic2.core.utils.collection.CollectionUtils;

@prufaraSS sounds like the version corrupted as you downloaded it.
That can happen from time to time.
Just redownload it.

@Speiger Nothing changed...

And also doesn't seem to have this lib in repo in both 'master' and '1.19.x' branches:

Ok, i already closed the original issue. Opening a second one is not a good idea.
I comment on closed ones too.

Anyways. CollectionUtils is simply a Utils class for creating commonly used collections so we don't have to ask: "What implementation should i use inside the team" we just have 1 class.
But you knew that already if you checked out what this is being used for.

Anyways.
You can't port IC2Classic by porting the API.