fix: ensure file exists before before telling browserify about it

[dariocravero]

There seems to be an issue with dependencies being reported as files like babelHelpers that creep up here. I don't know if there's a better solution but checking that the file exists seems like a good short-circuit for now. What do you think @nolanlawson? cc @amymc who spotted it first

[nolanlawson]

Thank you!