Support fallbacks

[kettanaito]

Expected behavior

• Each alias can support a string[] declaration of paths, where each path represents a lookup location of the resolved module, taking priority from left to right.

// alias.config.js
module.exports = {
  'utils/*': ['src/utils/*', 'src/utils/internal/*']
}

Considering this import:

import compose from 'utils/compose'

Module path resolution happens as follows:

• If src/utils/compose.js exist, it's resolved.
• If src/utils/compose.js does not exist, while src/utils/internal/compose.js does, the latter gets resolved.
• If neither of the paths exist, it's a module resolution error (module not found).

Mentions

TypeScript, webpack, and Jest support fallbacks (array of paths) natively. Rollup support would require writing a custom resolver function that looks up the given paths in the filesystem before resolving.