race-condition introduced by #54788 in the `import_module` function

[vtjnash]

The test failure in #56291 looks like it is running into a race-condition introduced by #54788 (@Keno) in the import_module function. There is an existing comment in the import_module function mentioning need to handle this parallel using assignment carefully, which seems to have regressed.

package loading: Error During Test at /usr/home/julia/.buildkite-agent/builds/freebsd13-amdci6-3/julialang/julia-master/julia-c05e8f6358/share/julia/test/threads_exec.jl:1124
2024-10-22 19:55:19 UTC	  Got exception outside of a @test
2024-10-22 19:55:19 UTC	  TaskFailedException
2024-10-22 19:55:19 UTC	
2024-10-22 19:55:19 UTC	      nested task error: importing TestPkg into Main conflicts with an existing global
2024-10-22 19:55:19 UTC	      Stacktrace:
2024-10-22 19:55:19 UTC	       [1] eval
2024-10-22 19:55:19 UTC	         @ ./boot.jl:459 [inlined]
2024-10-22 19:55:19 UTC	       [2] (::var"#229#230"{Event, Channel{Bool}})()
2024-10-22 19:55:19 UTC	         @ Main /usr/home/julia/.buildkite-agent/builds/freebsd13-amdci6-3/julialang/julia-master/julia-c05e8f6358/share/julia/test/threads_exec.jl:1137
2024-10-22 19:55:19 UTC	  Stacktrace:
2024-10-22 19:55:19 UTC	    [1] sync_end(c::Channel{Any})
2024-10-22 19:55:19 UTC	      @ Base ./task.jl:601
2024-10-22 19:55:19 UTC	    [2] macro expansion
2024-10-22 19:55:19 UTC	      @ ./task.jl:634 [inlined]
2024-10-22 19:55:19 UTC	    [3] macro expansion
2024-10-22 19:55:19 UTC	      @ /usr/home/julia/.buildkite-agent/builds/freebsd13-amdci6-3/julialang/julia-master/julia-c05e8f6358/share/julia/test/threads_exec.jl:1132 [inlined]
2024-10-22 19:55:19 UTC	    [4] macro expansion
2024-10-22 19:55:19 UTC	      @ /usr/home/julia/.buildkite-agent/builds/freebsd13-amdci6-3/julialang/julia-master/julia-c05e8f6358/share/julia/stdlib/v1.12/Test/src/Test.jl:1707 [inlined]
2024-10-22 19:55:19 UTC	    [5] macro expansion
2024-10-22 19:55:19 UTC	      @ /usr/home/julia/.buildkite-agent/builds/freebsd13-amdci6-3/julialang/julia-master/julia-c05e8f6358/share/julia/test/threads_exec.jl:1125 [inlined]
2024-10-22 19:55:19 UTC	    [6] macro expansion
2024-10-22 19:55:19 UTC	      @ /usr/home/julia/.buildkite-agent/builds/freebsd13-amdci6-3/julialang/julia-master/julia-c05e8f6358/share/julia/stdlib/v1.12/Test/src/Test.jl:1707 [inlined]
2024-10-22 19:55:19 UTC	    [7] top-level scope
2024-10-22 19:55:19 UTC	      @ /usr/home/julia/.buildkite-agent/builds/freebsd13-amdci6-3/julialang/julia-master/julia-c05e8f6358/share/julia/test/threads_exec.jl:32
2024-10-22 19:55:19 UTC	    [8] include
2024-10-22 19:55:19 UTC	      @ ./Base.jl:582 [inlined]
2024-10-22 19:55:19 UTC	    [9] exec_options(opts::Base.JLOptions)
2024-10-22 19:55:19 UTC	      @ Base ./client.jl:329
2024-10-22 19:55:19 UTC	   [10] _start()
2024-10-22 19:55:19 UTC	      @ Base ./client.jl:558