Doesn't work properly on Windows

[maxdavidson]

There appears to be some path normalization issue on Windows, causing all tests to fail and external source maps to not be picked up

https://travis-ci.org/github/maxdavidson/rollup-plugin-sourcemaps/builds/681295409

detects files with source maps › sourceMap: false, inlineSourceMap: true, inlineSources: false
    expect(received).toStrictEqual(expected) // deep equality
    - Expected  - 1
    + Received  + 1
      Array [
    -   "C:\\Users\\travis\\build\\maxdavidson\\rollup-plugin-sourcemaps\\src\\index.ts",
    +   "C:/Users/travis/build/maxdavidson/rollup-plugin-sourcemaps/src/index.ts",
      ]
 
detects files with source maps › sourceMap: true, inlineSourceMap: false, inlineSources: true
    expect(received).toStrictEqual(expected) // deep equality
    - Expected  - 1
    + Received  + 1
      Array [
    -   "C:\\Users\\travis\\build\\maxdavidson\\rollup-plugin-sourcemaps\\src\\index.ts",
    +   "C:/Users/travis/build/maxdavidson/rollup-plugin-sourcemaps/src/index.js",
      ]