allow extensions to trigger from packages in [deps] (#54009)
There is a use case where you have a weak dependency (for one of your
extensions) that is misbehaving and you quickly want to try debug that
issue. A workflow that feels reasonable for this could be:
```
pkg> dev WeakDependency
# do some fixes in this dependency
julia> using Package, WeakDependency
# this loads the extension of Package triggered by loading WeakDependency
# check that things work ok now
```
This doesn't work right now for two reasons:
1. Doing the `dev WeakDependency` will add the dependency to `[deps]`
but not remove it from `[weakdeps]` which means you all of a sudden are
in the scenario described in
https://pkgdocs.julialang.org/v1/creating-packages/#Transition-from-normal-dependency-to-extension
which is not what is desired.
2. The extension will not actually load because you can right now only
trigger extensions from weak deps getting loaded, not from deps getting
loaded.
Point 1. is fixed by https://github.com/JuliaLang/Pkg.jl/pull/3865
Point 2. is fixed by this PR.
