Use isbindingresolved when checking imported packages (#21580)

(cherry picked from commit d762038)

Author: ararslan

base/pkg/entry.jl

@@ -541,7 +541,7 @@ function resolve(
                 info("$(up)grading $pkg: v$ver1 => v$ver2")
                 Write.update(pkg, Read.sha1(pkg,ver2))
                 pkgsym = Symbol(pkg)
-                if isdefined(Main, pkgsym) && isa(getfield(Main, pkgsym), Module)
+                if Base.isbindingresolved(Main, pkgsym) && isa(getfield(Main, pkgsym), Module)
                     push!(imported, "- $pkg")
                 end
             end

test/pkg.jl

@@ -489,6 +489,16 @@ temp_pkg_dir() do
         @test isempty(Pkg.dependents("Example"))
         @test isempty(Pkg.dependents("Example.jl"))
 
+        @test_warn s -> !contains(s, "updated but were already imported") begin
+            Pkg.add("Iterators")
+            Pkg.update("Iterators")
+        end
+
+        # Do it again, because the above Iterators test will update things prematurely
+        LibGit2.with(LibGit2.GitRepo, metadata_dir) do repo
+            LibGit2.reset!(repo, LibGit2.GitHash(old_commit), LibGit2.Consts.RESET_HARD)
+        end
+
         @test_warn ("INFO: Installing Colors v0.6.4",
                     "INFO: Installing ColorTypes v0.2.2",
                     "INFO: Installing FixedPointNumbers v0.1.3",