workspace should provide a way to detect already-initialized modules

[stevengj]

For some C libraries (see JuliaIO/Blosc.jl#19), it is not safe to initialize them more than once, nor do we want to register an atexit hook more than once. However, if you do a workspace() then re-import the module, its __init__ is called a second time. It would be good to have a documented way to check whether a module was already initialized in a previous workspace.

(It's not sufficient to look at LastMain because the user could call workspace() twice.)

For example, there could be some kind of module_init_called(modulename) function that looks up a module in a global list of initialized modules that persists across workspace calls.

[yuyichao]

Milestone moved from #18646

[maleadt]

Related: calling reload($PKG) does the same, so it would be nice for a fix to span both cases.

[JeffBezanson]

Fixed by #17997. Now top-level modules are kept in a hidden internal table, so using X in a new Main module just re-references the same module. An explicit reload is needed to actually load again.