Support [sources] section in Project.toml for Julia 1.11

[j-fu]

Hi, in Julia 1.11, Project.toml can have a [sources] section which allows to specify a path or an url for a package from the [deps] section, this was announced at JuliaCon, see also
https://github.com/JuliaLang/Pkg.jl/blob/master/CHANGELOG.md#pkg-v111-release-notes

Could this be somehow supported in Pluto ? Something like

package_source(pkgname, pkgsource)

An alternative might be some tools which help to manipulate notebook environments. May be they even are already there but may be not marked as public interface.

[fonsp]

Thanks for the suggestion! [sources] is a really good fit for Pluto!

This gave me the idea for #3099 :)

[fonsp]

sources should already be kind-of supported in Pluto.jl (sources will be used if they are specified), but there are lots of places in our code where we still need to support sources, like when Pluto automatically adds/removes/updates packages.

One issue I found is JuliaLang/Pkg.jl#4086, it won't be possible to properly support sources until this is fixed.