[pan-domain-node] Circular import dependency

[mbarton]

The pan-domain-lambda build shows a circular import dependency in the lib. We should fix.

(!) Circular dependencies

node_modules/@guardian/pan-domain-node/dist/src/api.js -> node_modules/@guardian/pan-domain-node/dist/src/panda.js -> node_modules/@guardian/pan-domain-node/dist/src/api.js

node_modules/@guardian/pan-domain-node/dist/src/api.js -> node_modules/@guardian/pan-domain-node/dist/src/panda.js -> /home/travis/build/guardian/pan-domain-lambda/node_modules/@guardian/pan-domain-node/dist/src/api.js?commonjs-proxy -> node_modules/@guardian/pan-domain-node/dist/src/api.js