(1.10) ] precompile does not understand extension triggers in [deps]

[topolarity]

You also hit that error if you only add a symmetric extension Parent → DepWithParentExtExt:

diff --git a/test/project/Extensions/Parent.jl/Project.toml b/test/project/Extensions/Parent.jl/Project.toml
index a79ec8859b..9119864a47 100644
--- a/test/project/Extensions/Parent.jl/Project.toml
+++ b/test/project/Extensions/Parent.jl/Project.toml
@@ -4,3 +4,6 @@ version = "0.1.0"

 [deps]
 DepWithParentExt = "8a35c396-5ffc-40d2-b7ec-e8ed2248da32"
+
+[extensions]
+DepWithParentExtExt = "DepWithParentExt"
diff --git a/test/project/Extensions/Parent.jl/ext/DepWithParentExtExt.jl b/test/project/Extensions/Parent.jl/ext/DepWithParentExtExt.jl
new file mode 100644
index 0000000000..c9232efcbd
--- /dev/null
+++ b/test/project/Extensions/Parent.jl/ext/DepWithParentExtExt.jl
@@ -0,0 +1,6 @@
+module DepWithParentExtExt
+
+using DepWithParentExt
+using Parent
+
+end

That diff introduces an extension "cycle" of the kind that doesn't exist on 1.11, but is a cycle on 1.10.

We don't even get to print our warning though:

./julia --project=test/project/Extensions/tempenv -q
(tempenv) pkg> dev ./test/project/Extensions/DepWithParentExt.jl/ ./test/project/Extensions/Parent.jl/
   Resolving package versions...
    Updating `~/repos/julia/test/project/Extensions/tempenv/Project.toml`
  [8a35c396] + DepWithParentExt v0.1.0 `../DepWithParentExt.jl`
  [58cecb9c] + Parent v0.1.0 `../Parent.jl`
    Updating `~/repos/julia/test/project/Extensions/tempenv/Manifest.toml`
  [8a35c396] + DepWithParentExt v0.1.0 `../DepWithParentExt.jl`
  [58cecb9c] + Parent v0.1.0 `../Parent.jl`

(tempenv) pkg> precompile
ERROR: KeyError: key "DepWithParentExt" not found
Stacktrace:
  [1] getindex
    @ ./dict.jl:498 [inlined]
  [2] precompile(ctx::Pkg.Types.Context, pkgs::Vector{Pkg.Types.PackageSpec}; internal_call::Bool, strict::Bool, warn_loaded::Bool, already_instantiated::Bool, timing::Bool, _from_loading::Bool, kwargs::@Kwargs{io::Base.TTY})
    @ Pkg.API ~/repos/julia/usr/share/julia/stdlib/v1.10/Pkg/src/API.jl:1168
  [3] precompile(pkgs::Vector{Pkg.Types.PackageSpec}; io::Base.TTY, kwargs::@Kwargs{})
    @ Pkg.API ~/repos/julia/usr/share/julia/stdlib/v1.10/Pkg/src/API.jl:159
  [4] precompile(pkgs::Vector{Pkg.Types.PackageSpec})
    @ Pkg.API ~/repos/julia/usr/share/julia/stdlib/v1.10/Pkg/src/API.jl:148
  [5] precompile(pkgs::Vector{String})
    @ Pkg.API ~/repos/julia/usr/share/julia/stdlib/v1.10/Pkg/src/API.jl:147

Originally posted by @topolarity in #56675

Pkg is trying to look-up extension triggers in just [weakdeps] instead of both [deps] and [weakdeps]. This is not an issue on 1.11+, since the new ExplicitEnv code properly does the look-up properly

This is an important fix to get into 1.10, since our fix for JuliaLang/julia#56204 can sometimes force you to add triggers on [deps] instead of just [weakdeps]