Proposal: Potfile dependencies

[vknabel]

Introduction
Add the ability to declare dependencies within the Potfile.

Motivation
Every programming language needs a package manager. So should Lithia.

Proposed Solution

import pot.deps

deps.git "tests", "https://github.com/vknabel/lithia-tests.git", deps.Branch "main"

In the beginning only Branch and explicit Versions should be supported to reduce the implementation overhead.
Subsequent or nested potfiles might be ignored in the beginning.