fix(expiring-todo-comments): support monorepos

[mmkal]

Fixes #403
Maybe related: #1871

The basic change: pass the file-being-linted dirname rather than cwd to read-pkg-up. Using cwd meant it would find a monorepo's root package directory. Basically this:

- packageResult = readPkgUp.sync({normalize: false});
+ packageResult = readPkgUp.sync({normalize: false, cwd: dirname});

For review, worth viewing without whitespace, since a few functions were indented to live inside a helper which knows about the file-being-linted's directory name: https://github.com/sindresorhus/eslint-plugin-unicorn/pull/2159/files?w=1

I haven't added tests because I didn't see any that were actually exercising read-pkg-up so far, but maybe I missed something?

This could introduce new errors, or make others go away, so I think it qualifies as a breaking change. I think it makes sense for the dependency to depend on the package.json nearest the file being linted though.