Docgen loads unnecessary jsii files

[rix0rrr]

This is work that came out of: #1854

Currently, docgen loads all .jsii files it can find.

That's wrong, it should do the following:

• Load the very first assembly, potentially using a permissive indicator (point to a file, point to a directory with a .jsii file, just take a module name and load it from top-level node_modules, take nothing and load from the current directory).
• From that assembly, load its dependencies, resolving those strictly using require.resolve()
• Recurse on step 2.