refactor: move artifact types under artifact package to avoid import cycles

[knqyf263]

Description

Currently, ArtifactReference is defined in the types package under pkg/fanal/types, which holds a reference to core.BOM internally. Since PkgIdentifier is also defined in the types package under pkg/fanal/types, attempting to use PkgIdentifier from core.BOM results in a circular dependency. To work around this, a similar struct to PkgIdentifier is redefined in core.BOM now.

This is not ideal. This PR proposes moving ArtifactReference to the artifact package under pkg/fanal/artifact to resolve the circular dependency and enable the reuse of PkgIdentifier. Also, it refactors the growing number of package-related structs into a separate file (pkg/fanal/types/package.go) to improve code readability and maintainability. This PR doesn't change any logic.

Checklist

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• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).