Respect files section of private package

[zbigg]

While we more or less respect .npmignore of private packages, we should also properly respect files section "local" packages.

Example in monorepos built withyarn workspaces:
$ yarn workspaces run build
$ copy-production-deps ./some-backend some-backend/dist

copy-production-deps should be able to acuire only "publshed" files from all workspaces referenced by "some-backend" package. Now it will will copy all not .npmignored files.