Precompiled gensyms are not unique

[cstjean]

On Julia 1.7, the first gensym you get at the REPL is going to be Symbol("##274"), even if you've imported packages that contain that symbol. Witness:

module TestPkg
g = gensym()
end # module

then

julia> using TestPkg
[ Info: Precompiling TestPkg [9db6ce8c-046d-11e9-3d6b-c5272c17d34a]

julia> TestPkg.g
Symbol("##274")

julia> TestPkg.g == gensym()
true

Obviously this is a pretty rare problem, but for context, we thought that

@gensym f
@eval MyModule $f() = ...

couldn't possibly overwrite a method in MyModule. Turns out this is false because of this issue. It's been a very painful bug to figure out.

Semi-related: #18172

[vtjnash]

They are unique to the containing module only, and we possibly cannot do better