execution fails MODULE_NOT_FOUND

Hey ds300,

I am getting the following issue in CI

```
$ patch-package
internal/modules/cjs/loader.js:985
throw err;
^
Error: Cannot find module './dist/index.js'
Require stack:
- /home/node_modules/.bin/patch-package
at Function.Module._resolveFilename (internal/modules/cjs/loader.js:982:15)
at Function.Module._load (internal/modules/cjs/loader.js:864:27)
...
code: 'MODULE_NOT_FOUND',
requireStack: [ '/home/node_modules/.bin/patch-package' ]
```

I think the root cause stems from cachings the dependencies. The actual cause I assume is that the symlink in the `node_modules/.bin` is no longer a symlink and that the relative import from it therefore can no longer work.

As I have not encountered this issue with other cached packages I had a look into several of their `.bin` linked files and have seen that relative links are usually not used.

**I think this could be easily solved by changing the `require` here: https://github.com/ds300/patch-package/blob/master/index.js#L3 to `require("patch-package")`.**

This will correctly link to the same file as per the `package.json` `main` field.

Do you think that makes sense? Would you accept an PR for this or make the change yourself?