change req to resolve based on file path rather than directory (#231)

We previously had a fabricated file name (`_`) to forcefully resolve a
given directory path to a file path, so `createRequire` would be happy.

Since we now pass the file path every time, we no longer need this.

In future, if we need to `req` a directory, we should resolve the path
at the call-site rather than inside `req`.

src/req.js

@@ -1,10 +1,9 @@
 // eslint-disable-next-line node/no-deprecated-api
 const { createRequire, createRequireFromPath } = require('module')
-const path = require('path')
 
 function req (name, rootFile) {
   const create = createRequire || createRequireFromPath
-  const require = create(path.resolve(rootFile, '_'))
+  const require = create(rootFile)
   return require(name)
 }