cmd/compile: bug in cmd/compile/internal/noder.(*reader).pkgDecls

[prattmic]

#!stacks
"<insert predicate here>"

Issue created by stacks.

This stack ff8DlA was reported by telemetry:

• compile/bug
• cmd/compile/internal/base.FatalfAt:+3
• cmd/compile/internal/base.Fatalf:=195
• cmd/compile/internal/base.Assert:=242
• cmd/compile/internal/noder.assert:=15
• cmd/compile/internal/noder.(*reader).pkgDecls:+26
• cmd/compile/internal/noder.(*reader).pkgInit:+9
• cmd/compile/internal/noder.unified:+14
• cmd/compile/internal/noder.LoadPackage:+50
• cmd/compile/internal/gc.Main:+140
• main.main:+12
• runtime.main:+125
• runtime.goexit:+0

cmd/compile@go1.23.4 go1.23.4 darwin/arm64 (2)

Another export data case.

cc @golang/compiler @timothy-king @griesemer