relative paths in sources are not made relative to output file after being resolved.

in
```
a/dist/index.js
a/dist/index.map ../src/index.ts
a/src/index.ts
b/dist/index.js
b/dist/index.map  ../src/index.ts
b/src/index.ts
```
out
```
bundle.js
bundle.map  sources: ['../src/index.ts', '../src/index.ts']
```

expected
```
bundle.js
bundle.map  sources: ['a/src/index.ts', 'b/src/index.ts']
```
