esm implementation is breaking for some node versions
Node.js versions between 13.0.0 and 13.6.0 still had larger bugs in their ESM implementations and the esm wrapper breaks requiring `koa` the regular way on those runtimes.

```
❯ node           
Welcome to Node.js v13.0.0.
Type ".help" for more information.
> require('koa')
Thrown:
Error: Package exports for '/Users/panva/repo/provider/node_modules/koa' do not define a valid '.' target
    at resolveExportsTarget (internal/modules/cjs/loader.js:455:13)
    at resolveExports (internal/modules/cjs/loader.js:388:16)
    at Function.Module._findPath (internal/modules/cjs/loader.js:487:20)
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:782:27)
    at Function.Module._load (internal/modules/cjs/loader.js:688:27)
    at Module.require (internal/modules/cjs/loader.js:850:19)
    at require (internal/modules/cjs/helpers.js:74:18) {
  code: 'MODULE_NOT_FOUND'
}
> 
```

```
❯ node          
Welcome to Node.js v13.6.0.
Type ".help" for more information.
> require('koa')
Uncaught:
Error: No valid exports main found for '/Users/panva/repo/provider/node_modules/koa'
    at resolveExportsTarget (internal/modules/cjs/loader.js:625:9)
    at applyExports (internal/modules/cjs/loader.js:502:14)
    at resolveExports (internal/modules/cjs/loader.js:551:12)
    at Function.Module._findPath (internal/modules/cjs/loader.js:657:22)
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:960:27)
    at Function.Module._load (internal/modules/cjs/loader.js:855:27)
    at Module.require (internal/modules/cjs/loader.js:1033:19)
    at require (internal/modules/cjs/helpers.js:72:18) {
  code: 'MODULE_NOT_FOUND'
}
```

I've ran into this in the past and know that for those node versions this is an unfixable problem so I don't have a suggestion for a way forward, just wanted to let you know.

For me this means changing my CI test matrix to test "low 13" with 13.7.0 instead 13.0.0 and have my fingers crossed.

This was merged in #1474