Some Manifold objects fail to import into Manifold

[kintel]

When importing files into Manifold, we first convert to a polygon soup, and then try to repair it.
This doesn't work for certain types of manifolds, specifically meshes which touch itself in a single face:

https://github.com/openscad/openscad/blob/98c132516a80312604c346557af52fbe7091638f/tests/data/scad/3D/misc/polyhedron-self-touch-face.scad

To fix this, it would be interesting to try to bypass the polygon soup step in cases where the imported object is a valid manifold.
This also happens with polyhedron() definitions, not just imports.