Hi,

'Installation library' is the terminology used on the OpenScad wiki and described as:
"The Installation library location is the libraries directory under the directory where OpenSCAD is installed."
https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/Libraries

So this use <MCAD/2Dshapes.scad> is valid in openscad,
in SolidPython import_scad("MCAD/2Dshapes.scad") is not valid, it fails with the error

Locations searched were: [PosixPath('MCAD/2Dshapes.scad'), PosixPath('/home/user/.local/share/OpenSCAD/libraries/MCAD/2Dshapes.scad')]

indicating that the installation library path is not search at all.

This is occuring with solidpython 1.1.1 on arch linux