- this currently causes a bug on Windows with mixed path separators
- it returns a POSIX backslash path for __dirname, and then adds a
forward slash from the directorySeparator, causing mixed separators
- TS uses `/` internally and lets the host convert if needed, so
I assume this should be normalized to all forward slashses instead
- example of the bug from my tests:
```
Expected: "D:\\a\\rollup-plugin-typescript2\\rollup-plugin-typescript2\\node_modules\\typescript\\lib\\lib.d.ts"
Received: "D:\\a\\rollup-plugin-typescript2\\rollup-plugin-typescript2\\node_modules\\typescript\\lib/lib.d.ts"
```
- every other use of __dirname in the codebase seems to be normalized
except for this one
- could use normalizeSlashes for this, but I figure combinePaths is
more appropriate since that will handle it and combine properly as
well without any `+ directorySeparator +` stuff
