Use serverless servicePath in loadSource

[larte]

What did you implement:

Added resolving of full path in loadSource .

When using a more exotic deployment mechanism, yarn workspaces or extending the serverless CLI , cwd might not be resolved to service root automatically.

How did you implement it:

Combine servicePath from serverless config with relative sourceFilePath.

How can we verify it:

• run the plugin normally.

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