feat: Aliasing a npm module without 'npm:'

[tleunen]

The npm: prefix wasn't really necessary because we can detect if the final path is a local file (starting with a dot), or a path to a npm module.

The following example will redirect all underscore calls to lodash.

{
  "alias": {
    "underscore": "lodash"
  }
}

[codecov-io]

Current coverage is 98.18% (diff: 100%)

Merging #73 into master will not change coverage

Diff Coverage	File Path
•••••••••• 100%	src/mapToRelative.js
•••••••••• 100%	src/index.js

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