make loading work with a entryfile entry in the manifest file

[KristofferC]

also soft deprecate the path entry in project file in favour of using entrypath instead

Fixes the Julia Base part of #53937

[KristofferC]

Pkg PR is at JuliaLang/Pkg.jl#3850.

[Keno]

Haven't tried it out, but from my understanding of the loading code this is reasonable. I do like the path/entrypath disambiguation. Probably should have docs somewhere though.

[KristofferC]

I added an entry to the docs. Please see if it is understandable.

[Keno]

Docs look fine to me.

[KristofferC]

I'm thinking entryfile is better than entrypath. It is a little bit strange to have path and entrypath but having path and entryfile makes sense. (entrypath is then naturally joinpath(path, entryfile).)