Transform plugin incompatible with permalink

[nathonius]

Repro here: https://github.com/nathonius/11ty-demo/tree/main/4-images

Discord thread about this issue

When a urlPath option is NOT provided to the transform plugin, but the template including the image has a permalink attribute that is a filename, not a directory, the image source is incorrectly uses the full html path as if it was a directory. For example:

/blog/posts/Blog Post.md

---
permalink: blog/posts/blog-post.html
---
![alt text](./images/my-img.png)

results in

<img src="/blog/posts/blog-post.html/BshP-bOYIK-710.jpeg" alt="alt text" width="710" height="618">

I'd expect it to understand that the permalink is the full path to the file in this case, not a directory with an index file. This should result in the following (which correctly links to the file):

<img src="/blog/posts/BshP-bOYIK-710.jpeg" alt="alt text" width="710" height="618">

Either removing the permalink (or using a directory permalink) or using a urlPath in the plugin config bypasses the issue.