Add doctor command to diagnose common problems and print information for further debug

Having more than an instance of scala-cli on PATH creates problems with updates, since an old version might appear sooner on PATH and get executed instead of the newer version!

How should it work:

The user runs scala-cli doctor

Then, if there are any duplicates on PATH it gets reported to the user!

One implementation strategy is

• first seeing if type -ap scala-cli shows duplicates;
• then if so, we run ls -l on the result to make sure they are not all symlinks to the same location.
• Then if they were not symlinks, we cat the contents of them to see if they run the same script. (this is tricky)
• Now if they weren’t scripts executing the same thing, we produce a PATH duplicates error

Alternatively, this information can be provided by running the update, about, or version commands.

The team decided that the PATH duplicate part of issue #543 should be dealt with as an independent separate issue. It is also related to issue #670

[mtk]

re: coursier - i get your point. strategically, i think it should be delivered by coursier, though. if you’re going to pay the climbing-the-learning-curve price for only one tool (e.g. as a student in a class), installing coursier manually gets you more for your money. chris kipp asked me to open a coursier issue requesting that scala-cli move out from under “install --contrib” to just “install”. haven’t done it yet but i will.

re: doctor - that’s a great list of things to-do. speaking only for myself, that won’t fit into two hours. for some reason i didn’t see your post until today. however, i’d be happy to work on this well after the spree ends.

[SethTisue]

Let’s call it doctor, in imitation of Homebrew (and also Metals, according to @romanowski).