`create_sysimage` gets confused by unrelated projects matching `JULIA_PROJECT`

This is on julia 1.8.4, PC.jl 2.1.2.

A MWE can be:
* `$ mkdir /tmp/jl_project; cd /tmp/jl_project`
* `$ julia --project=.`
* `]generate XYZ`; `]dev ./XYZ`
* `$ mkdir image; cd image; julia --startup-file=no`
* `]activate --temp`
* `]add InlineTest`
* `ENV["JULIA_PROJECT"] = "@."`
* `using PackageCompiler; create_sysimage(sysimage_path="./image.so")`

This results in the following error:
```
julia> using PackageCompiler; create_sysimage(sysimage_path="./image.so")
0 dependencies successfully precompiled in 0 seconds

ERROR: The following 1 direct dependency failed to precompile:

XYZ [12bc79f3-ea85-4693-851d-069bcf1ea768]

Error: Missing source file for XYZ [12bc79f3-ea85-4693-851d-069bcf1ea768
Stacktrace:
[...]
```
It doesn't seem correct to consider the project at "/tmp/jl_project" inside `create_sysimage`, as the current active project is something else.

Reproduce the listed MWE with Julia 1.8.4, PC.jl 2.1.2, and the create_sysimage entry point while JULIA_PROJECT is set to "@.". Compare behavior with and without the unrelated active project. Done means create_sysimage no longer treats that project as a direct dependency and the MWE completes without the missing-source-file error.

Written by the indexing model from the issue text.