Prune versions not referenced in juliaup.json with --prune-orphans

[christiangnrd]

I took a look at #1129 and it seems like if a direct download channel was deleted and the .juliaup folder was not, it would forever be ignored by the gc command. I attempt to address it by adding a --prune-orphans flag similar to the --prune-linked flag that will remove any folder that starts with "julia-" that is not referenced by the "juliaup.json" config file.

Name is definitely open for suggestions, and I'd love suggestions to make my code more idiomatic. At the moment it feels clunky.

If this approach is accepted, I'll write a test.

I also added some clarifying comments to the SystemChannel code to make it easier to understand.

[ket395]

Really appreciate the comments and clean simple code. I ran into this issue. But didn't have time to figure out a way to contribute a fix. My current workaround is to just not download any new versions.

Before I forget, a suggestion for a different PR would be to add the inverse of this flag to juliaup to preserve old behaviour by default because most probably someone is depending on it. As surprising or as obvious as it is.

I saw a comic about an exaggerated version of this phenomena in software engineering on a comic but I forget the link. I suspect it was XKCD and it was about a bug fix in the even more older no ancient, religiously venerable Emacs software system with an obscure oldfangled help shortcuts and docs. Its configured by using a dialect of Lisp, Emacs Lisp which allows modern bundles of it like Spacemacs. But I may be absolutely wrong about the comic.

I wish I knew about this PR before. Or that someone had reviewed it for idiomatic Rust lang code.

PS - Can't think of any other issue with this commit. Will check the PR later, possibly.

Manish, check out my kind of oldfangled, kinda newfangled, kinda cool, inevitably thought-provoking profile.