Bug: Shouldn't fall back to pkg name in the absence of output flag & pkg main

[rschristian]

Packages that don't use the output flag or have a "main" in their package.json have their preferred filename output ignored.

For example, say I have the following package:

{
  "name": "foo",
  "exports": {
    ".": "./dist/bar.js",
    ...
  }
}

Unfortunately, because of the replaceName() here, the desired output is replaced by the package name instead.

https://github.com/developit/microbundle/blob/b1a637486234a2ae784ccf0c512321e2d3efef7c/src/index.js#L297-L303

https://github.com/developit/microbundle/blob/b1a637486234a2ae784ccf0c512321e2d3efef7c/src/index.js#L255-L260

mainNoExtension traces back to here, which defines a couple of fallbacks.

https://github.com/developit/microbundle/blob/b1a637486234a2ae784ccf0c512321e2d3efef7c/src/index.js#L219-L225

I can't really say that I understand why this was done in the first place, so don't want to go "fixing" it as there might be some relying on this. To me, it seems like that in the absence of the output flag, these fields should be read and used exactly, no replacing.

mainNoExtension seems like it should be the last of the fallbacks, not something that overwrites valid entries & chosen names.