feat: add support for installing instead of symlinking file: dependencies

[nlf]

this adds an installLinks flag to arborist, which when set instructs arborist to pack and extract file: links rather than creating a symlink to their directory. this also causes us to reify the dependencies of the directory.

a follow up pull request will add the config definition and documentation for the flag. i changed my mind about a separate pull request, i'll be adding some commits to this one instead.

for #2339
for npm/rfcs#150

[ljharb]

I'd really prefer if link: is for symlinks and file: is for copying files, like it used to be before npm 5 broke it, but another way to get file copying is still great

[nlf]

I'd really prefer if link: is for symlinks and file: is for copying files, like it used to be before npm 5 broke it, but another way to get file copying is still great

that would be great, but also a breaking change. i'd personally really like to see npm@9 go back to pack+install for file: deps and require the use of npm link to create a link. this is an interim feature to unblock folks who depend on the old behavior without breaking folks who have already adjusted to the new.

[lukekarrys]

This looks great! 🎉

@nlf Does it close either #2239 or npm/rfcs#150?

[nlf]

@nlf Does it close either #2239 or npm/rfcs#150?

in theory, yes, but i didn't want it to automatically close the issues when it lands so i can follow up in both places

[ThisIsMissEm]

Also perhaps related to @ljharb's comment: npm/feedback#667