Hash out wtf `npx ./foo` commands do

[zkat]

There's a particular corner case with npx usage that hasn't been quite specced out to satisfaction: local directory specifiers.

note: directory in npm-package-arg lingo includes regular directories as well as any files that don't end in .tar, .tgz, or .tar.gz. I'll be using that terminology here, with specific disambiguation as necessary.

The biggest problem is that there's several different situations where these specifiers can pop up, and we need to know what to actually do with them. I'll put a table here with the current + proposed behavior for the various situations, and hopefully we can come up with something useful here.

I definitely consider the current behavior to be buggy because I think literally no one wants what npx does for this right now.

command	current behavior	do this instead
npx ./pkgdir	npm i -g ./pkgdir && pkgdir	node ./pkgdir/${pkg.bin or pkg.main or index.js}
npx ./otherdir	npm i -g ./otherdir && otherdir	Error: executing random directories makes no sense
npx ./foo.bin	npm i -g ./foo.bin && foo.bin	./foo.bin
npx ./foo.js	npm i -g ./foo.js && foo.js	node ./foo.js
npx ./foo (foo has #!/usr/bin/env node)	npm i -g ./foo && foo	node ./foo
npx -p pkg ./whatever	npm i -g pkg && ./whatever	Exactly the same as above, but with an npm i -g pkg first.