Have import look in pkg dir before working directory

[dcjones]

Currently import Foo will first look in the working directory for an appropriately named file before trying to load a package called Foo. This PR reverses that precedence. I don't know if that's the optimal solution, or what the all the consequences might be, but I hope I can provoke some thought about this.

There are various problems that the current behavior can cause. All of them are not obvious failures, and can be difficult, especially for new users to debug. Here's an example:

1. Make a named Markdown.jl
2. Have it import Markdown
3. Run julia Markdown.jl
4. Julia hangs forever.

This is especially egregious on case-insensitive file systems, where the file might be named markdown.jl, or if import Markdown doesn't appear directly in that file, but something it imports. Furthermore, it's easy to break working code just by accidentally introducing files named the wrong things (e.g. images.jl, distributions.jl) in the same working directory. Once more command line tools are written in Julia, this behavior could be a security risk as well, at least as long as there's no way to disable it.

Our current behavior is similar to Python's, but I don't think this is a Python feature that's worth emulating. It has been raised before as a python issue (a decade ago), but dismissed because it would break too many things to change.

It also comes up periodically here as well, for example: #9079

[JeffBezanson]

Partly related: #4600

[yuyichao]

Shouldn't this (looking in working directory) only be done for REPL?

[jiahao]

bump

Given that #9079 has been the source of several insidious bugs, I'd like to propose fixing this for 0.4.0.

[timholy]

I'm in favor.

[JeffBezanson]

superseded by discussion in #9079.