`esModuleInterop` was enabled in
e4164c8892
to help with importing from packages that use default exports.
Those changes were reverted in
https://github.com/apollographql/apollo-server/pull/1210
to work around a few reported issues. Those issues are no longer
relevant, so this commit re-enables `esModuleInterop`, and
updates all default imports to use the more common (standard)
import syntax.