Disallow multiple files of the same name in model registry

1. Disallow multiple files of the same name in package registry

2. Use file name as index could be possible

The following discussion from !165274 (merged) should be addressed:

• @sheldonled started a discussion: (+1 comment)

  question (non-blocking): Could we use the file name inside this loop as reference?

          <div v-for="upload in uploads" :key="upload.file.name" class="py-3 gl-border-b">