Primary motivation is documentation, but it will also hopefully simplify the code.
Also,
* Lower case the default output format names; this is in line with the custom ones (map keys) and how
it's treated all the places. This avoids doing `stringds.EqualFold` everywhere.
Closes#10896Closes#10620
Same logic as for `index.{js,ts...}` files applies; if both `index.esm.js` and `index.js` exists (unlikely), you need to use the name
with extension when importing, else the `index.js` will win.
Fixes#8631
This is in line with how both Node and ESBuild's native import resolver does it.
The ambiguous situations above were discovered trying to build AlpineJS v3.
Note that the above was never an issue if you used `./foo.js` and similar to import the component.
Fixes#8945
