include_dependency not fully supported

[fonsp]

Hi! I tried JET.jl on the Pluto.jl repository and it failed with this error message:

═════ 1 toplevel error found ═════
┌ @ /Users/fons/Documents/Pluto.jl/src/Pluto.jl:38 
│ SystemError: opening file or folder "/Users/Project.toml": No such file or directory
│ Stacktrace:
│  [1] _include_dependency(mod::Module, _path::String; track_content::Bool, path_may_be_dir::Bool)
│    @ Base ./loading.jl:2096
│  [2] _include_dependency
│    @ ./loading.jl:2084 [inlined]
│  [3] #include_dependency#1081
│    @ ./loading.jl:2129 [inlined]
│  [4] include_dependency(path::String)
│    @ Base ./loading.jl:2128
└─────────────────────

This is caused by the following line:

https://github.com/fonsp/Pluto.jl/blob/84b9f1fa7f97275fa3f935f2dbb4acdff421240c/src/Pluto.jl#L38

It looks like the relative path in include_dependency was resolved relative to my pwd() (/Users/fons), and not the file location.