Ignore modules in library search path

[nmichlo]

Sometimes it makes sense to group helper utilities together in in the library search path.

However, we don't always want these files to be loaded if for example the model itself is not used.

• The problem is that modelkit traverses and loads all python modules in the search path. It would be useful to be able to skip over specific files.

This could maybe be done with a common naming convention, for example prefixing files that should be ignored with some standard prefix, eg. library._my_module would be ignored and skipped over (this follows private convention, with the underscore indicating a module that should not be exposed publicly), while library.my_module would be traversed and loaded.