fix: prevent duplicate artifact assignment

[NoxHarmonium]

What did you implement:

Prevent duplicate artifact assignment, extracted some duplicate logic.

Closes #1242

How did you implement it:

Some minor refactoring so that code paths are not called twice redundantly (see issue #1242)

Old Coverage:

=============================== Coverage summary ===============================
Statements   : 95.92% ( 2566/2675 )
Branches     : 89.67% ( 530/591 )
Functions    : 98.18% ( 108/110 )
Lines        : 95.92% ( 2566/2675 )
================================================================================

New Coverage:

=============================== Coverage summary ===============================
Statements   : 95.95% ( 2584/2693 )
Branches     : 89.71% ( 532/593 )
Functions    : 98.19% ( 109/111 )
Lines        : 95.95% ( 2584/2693 )
================================================================================

How can we verify it:

1. Run sls package --verbose with individual packaging switched on.
2. Grep for "Setting artifact for function"
3. Check that each function is only mentioned once

I have tested this in my local project and it works for me.

Todos:

• Write tests
• Write documentation
• Fix linting errors
• Make sure code coverage hasn't dropped
• Provide verification config / commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: NO

[j0k3r]

Good job, just found a small typo 👍🏼

[NoxHarmonium]

Typo fixed and branch rebased!

[vicary]

LGTM