Suggestion: Warn on `using SomePkg`

[jakobnissen]

It's quite common to see people using using SomePkg at the top of their code. While this is somewhat idiomatic Julia for scripts, it's ill-adviced for packages and other long-lasting code that one might want a linter for. If SomePkg exports a new symbol, it can clash with an existing symbol and throw an error, thus breaking existing code.

This problem could be reduced if the linter automatically warned on using SomePkg instead of using SomePkg: SomeSymbol