fix: add guard for options.browser with no pkg.browser field

[keithamus]

This adds a fix for the problem described in #127 (comment)

[Rich-Harris]

Thanks, released 3.0.2 👍

[Haroenv]

Perfect!

[wKovacs64]

@keithamus Hmm, this broke my build. I get this as of 3.0.2:

[!] Error: Unexpected token
node_modules/axios/package.json (2:8)
1: {
2:   "name": "axios",
           ^
3:   "version": "0.17.1",
4:   "description": "Promise based HTTP client for the browser and node.js",

Not sure if it's related to how axios is using the browser field to replace a specific file or what. I can open a dedicated issue for tracking if you'd like.

[Haroenv]

I opened #130 which I think has the same origin @wKovacs64

[wKovacs64]

@Haroenv Oh, cool, didn't realize that was the same thing. Thanks.

[feross]

Yes, this broke my build too. The "browser" field is being completely ignored.

[Haroenv]

This pr introduced behaviour where only the main file is resolved, but all of the files in the browser field should be resolved as described in the map. I couldn't figure out how to make that change in this code