eslint-plugin-import: npm 7 breaks tests

As of npm 7’s promotion yesterday, there’s at least two problems breaking tests on master and every PR:

• peer dep issues. eslint 4 is incompatible with @typescript-eslint/parser. this is solved with --legacy-peer-deps (commit).
• when node_modules is present in the root, and was installed with npm 7, both npm 6 and 7 fail on a bunch of the resolver tests, and on a few of the root tests (presumably related). When node_modules is absent, or chmod a-x node_modules, or installed with npm 6, things are fine. (lockfile diff)

It’s being looked into. After a few days of this, I’ll probably try to downgrade npm on affected builds, but it might reflect an issue that npm 7 users will encounter, so I’d prefer to find a proper fix.