Allow importing a ZIP library package with multiple libraries inside

I'm core developer of the Blynk library.
It supports lot's of hardware types and boards (more than 60!), but it also means it has a bunch of dependency libraries.
We run into a situation, where it would be much easier to import our library, bundled with some dependencies in a single ZIP file.

For example, to be able to import all libraries from this ZIP:
https://github.com/blynkkk/blynk-library/releases/download/v0.3.7/Blynk_v0.3.7.zip

We could think about adding a special marker file in the root of the package, so IDE can reliably detect a multi-library package.

The issue names no file, test, or entry point. Begin by reproducing import of the linked Blynk_v0.3.7.zip in Arduino IDE 1.x and tracing the existing library-import path. Define how a root marker or equivalent identifies contained libraries, then verify that all libraries import without breaking single-library ZIP imports.

Written by the indexing model from the issue text.