Rule suggestion: no-external-imports (enforce deps.ts convention)

[gewoonwoutje]

I'd like to use a deps.ts and/or import map to load external imports (i.e. non-relative imports), and would like to enforce this convention. In other words, I'd like to block using imports from https://... sources in all files, except import maps and a deps.ts dependencies file (/ files excluded with an ignore pragma).

[hosseind88]

If nobody is already working on this PR, I'd like to work on it