feat(saveSourceMap): honor source folder hierarchy

[kwonoj]

This PR is following changes to #185 and improves behavior of saving sourcemap, by honoring appRoot and create relative path under sourcemapPath when generates sourcemap.

This behavior supports 2 usecases,

• if there are same file name under different paths (src/main/main.ts, src/renderer/main.ts) flattened sourcemap emission will overwrite one eventually
• sourcemap lookup based on sourceroot / maproot is based on relative path hierarchy of original source, so if someone try to utilize it they may need to reconstruct structures from flattened one.

Now with this PR by default it honors appRoot and construct same hierarchies, and if someone need to flatten sourcemap can do after compilation is done. (manually).