bam_model_extensions changes external path references in distributions

[izzyboris]

Description

If a model file is converted using the bam_model_extensions from #714 and it references any external resources by pathname (textures, etc.), the write_bam_file() that saves the file changes the relative paths of these models and they may point into the source directory. Ideally I think the relative path references in the .bam outputs from the converter should be left alone so they point to the texture relative to their current location.

Steps to Reproduce

• Create a model with an external texture (e.g., a .blend using blend2bam or .gltf)
• Put ".blend" or .gltf in the bam_model_extensions list
• Run bdist_apps
• The external texture path will get munged during the file write_bam_file() within commands.py:_model_to_bam and point at the source directory instead of a texture copied into the build directory.

Environment

• Operating system: Windows (probably happens in Linux too)
• System architecture: 64-bit Intel
• Panda3D version: 0.10.13
• Installation method: pip
• Python version (if using Python): 3.11
• Compiler (if using C++): N/A