Reject ambiguous module imports.

[mbaulch]

Fixes #4485.

[Araq]

Filenames need to be unique per Nimble-package. And one test now fails with:

module stdlib/system is already defined in /home/travis/build/nim-lang/Nim/lib/system.nim

[mbaulch]

The way Nimble operates shouldn't, AFAIK, have any bearing on the compiler. The failing test relies on nimscript (compiler/scriptconfig.nim). I'll look for a sensible way to check whether nimscript is affecting module imports, and ignore the fake system/os packages it provides. Do you think that might work?

Not very knowledgeable about Nimble or Nimscript I confess.

[mbaulch]

There we are. Done.

[Araq]

Sorry, but this is convoluted and wrong (Nimscript can cause the reload any kind of module really, not only system.nim).

Every skModule symbol has an skPackage owner, why not store the list of known modules in this skPackage and report if there is an ambiguity arising?

[mbaulch]

I agree it's convoluted. Thanks for the pointer about Nimscript. That's important!

Will open a new PR, as that fix will have little in common with this one.

[mbaulch]

Sorry, I was a bit hasty with closing. Each skModule has a unique skPackage owner. We'd have to keep a list of skPackage representatives (PSym), and point each skModule's owner to the correct one. I can't see that such a list already exists, does it?

Your point about Nimscript (and modules besides system.nim getting reloaded) still stands, of course.

[mbaulch]

Any module can now be reloaded. I think we're okay now.

[Araq]

Fixed it differently, sorry. :-)

[mbaulch]

That's okay. Your fix 9eb909b is more comprehensive, and possibly more efficient too.