New TS types fail to import koa
Thanks for merging types directly into the lib.

My dependency bot is trying to pull this update, but my `tsc` builds with yarn 2.4.1 (with pnp) fail on it for some reason.

```
> tsc -b tsconfig.build.json

.yarn/cache/koa-helmet-npm-6.1.0-5168096bc9-63a0ab6ded.zip/node_modules/koa-helmet/koa-helmet.d.ts:9:37 - error TS2307: Cannot find module 'koa' or its corresponding type declarations.
```

To be honest, I'm kinda stumped, the source looks just fine.

Node 14.16.0 x64 on windows 10

tsconfig,json (definitely irrelevant stuff omitted)
```
{
    "compilerOptions": {
        "composite": true,
        "esModuleInterop": true,
        "importHelpers": true,
        "module": "commonjs",
        "noUncheckedIndexedAccess": true,
        "strict": true,
        "target": "es2020"
    }
}
```

package.json (also trimmed down)
```
    "dependencies": {
        "koa": "2.13.1",
        "koa-compose": "4.1.0",
        "koa-compress": "5.0.1",
        "koa-helmet": "6.1.0",
        "koa-router": "10.0.0",
    },
    "devDependencies": {
        "@types/koa": "2.13.1",
        "@types/koa-compose": "3.2.5",
        "@types/koa-compress": "4.0.1",
        "@types/koa-router": "7.4.1",
        "tslib": "2.1.0",
        "typescript": "4.2.3"
    }
```

I can send you a copy of my whole (tiny) project if you can't troubleshoot it with this.