[REPL] fix lock ordering mistake in load_pkg (#56215)

Fixes #56206

(cherry picked from commit 1f935af)

Author: vtjnash

base/loading.jl

@@ -2044,6 +2044,7 @@ debug_loading_deadlocks::Bool = true # Enable a slightly more expensive, but mor
 function start_loading(modkey::PkgId, build_id::UInt128, stalecheck::Bool)
     # handle recursive and concurrent calls to require
     assert_havelock(require_lock)
+    require_lock.reentrancy_cnt == 1 || throw(ConcurrencyViolationError("recursive call to start_loading"))
     while true
         loaded = stalecheck ? maybe_root_module(modkey) : nothing
         loaded isa Module && return loaded

stdlib/REPL/src/Pkg_beforeload.jl

@@ -1,17 +1,16 @@
 ## Pkg stuff needed before Pkg has loaded
 
 const Pkg_pkgid = Base.PkgId(Base.UUID("44cfe95a-1eb2-52ea-b672-e2afdf69b78f"), "Pkg")
-const Pkg_REPLExt_pkgid = Base.PkgId(Base.UUID("ceef7b17-42e7-5b1c-81d4-4cc4a2494ccf"), "REPLExt")
 
 function load_pkg()
+    REPLExt = Base.require_stdlib(Pkg_pkgid, "REPLExt")
     @lock Base.require_lock begin
-        REPLExt = Base.require_stdlib(Pkg_pkgid, "REPLExt")
         # require_stdlib does not guarantee that the `__init__` of the package is done when loading is done async
         # but we need to wait for the repl mode to be set up
-        lock = get(Base.package_locks, Pkg_REPLExt_pkgid.uuid, nothing)
+        lock = get(Base.package_locks, Base.PkgId(REPLExt), nothing)
         lock !== nothing && wait(lock[2])
-        return REPLExt
     end
+    return REPLExt
 end
 
 ## Below here copied/tweaked from Pkg Types.jl so that the dummy Pkg prompt