Use PluginContext.emitFile to emit declarations (when possible)

[marijnh]

See issue #180

See this comment for an explanation of the logic that chooses between writeFile and emitFile.

[ezolenko]

Looks good, but it is better if declarations are either all written or all emitted. So if pluginOptions.useTsconfigDeclarationDir is true, use tsModule.sys.writeFile for everything, and if not, emit everything. It would be easy to document in readme then and there will be no surprises downstream. Things should either work or not work, worst failure mode is when some files suddenly don't get processed by downstream plugins just because folder structure changed.

To resolve naming conflicts in the second case, path separators can be replaced with underscores, so outdir/subdir/name.d.ts becomes outdir/subdir_name.d.ts.

[ezolenko]

See comment

[marijnh]

Always directly writing with useTsconfigDeclarationDir is a good idea. Implemented in the updated patch.

I don't quite get your suggestion to substitute underscores. We can emit subdirectories (they just have to have the shape foo/bar.d.ts, not ../bar.d.ts), and mangling file names would break imports of those files from other declaration files.

[ezolenko]

Looks good, thanks. I'll make a release sometime this week hopefully.