Importing an FBX file loses any texture assignments that point to broken texture paths

When importing an FBX file, it is common that some of the texture paths are broken. This is due to the lack of tooling to properly export relative paths for FBX textures.

A typical fallback is to look for the texture in the source directory, the one containing the source FBX file being imported.

In addition, completely discarding textures that are not found makes it impossible to fix the texture paths in a post-processing pass, as it would be usual in an asset ingest pipeline.

**Expected Behaviour**

Importing an FBX file with broken texture references will result in materials with the broken textures still assigned, so that it is possible to inspect the usd stage and proceed with fixing the paths.

**Optional Extension**

As a fallback, any textures with the same filename, present in the source folder, will be picked instead, but a warning will be logged so that the user is aware of the tentative fix.

**Actual Behaviour**

The texture is completely ignored and the material will be missing the texture assignment, making it impossible to reassign the texture at a later stage.