Module imports ignored via “browser” field cause absolute paths in source maps

[eelco]

Follow-up of #785, absolute paths can still happens with module imports.

Example from https://github.com/socketio/engine.io-client/blob/master/package.json#L69-L74:

  "browser": {
    "ws": false,
    "xmlhttprequest-ssl": "./lib/xmlhttprequest.js",
    "./lib/transports/websocket-constructor.js": "./lib/transports/websocket-constructor.browser.js",
    "./lib/globalThis.js": "./lib/globalThis.browser.js"
  },

Causes an absolute path in the source map:

"(disabled):/Users/eelco/Projects/Example/node_modules/engine.io-client/node_modules/ws/browser.js"