[CI] Simplify injection of local dependency

[sebsto]

There might be an opportunity to simplify the package dependency during CI.
At the moment, we're relying on the presence of a AWS_LAMBDA_LOCAL_DEPS environment variable and we dynamically edit the dependency in the Package.swift file.

There might be an opportunity to simplify this code and replace it with swift package edit commands in the CI script.
However, we should continue to support local development (when developing a new example or when developing the library itself, it is useful to have a mechanism to refer to the local version)

Ref: #532 (comment)