Isolate mainExecutableFile error to runAfter

[stretchkennedy]

The CustomAction LaunchedInstalledExe refers to a FileKey for a non-existent file, mainExecutableFile. Since it's only necessary for supporting --run-after, this commit stops it being defined unless runAfter is truthy.

This is not a fix for the underlying issue! However, it isolates the problem to users of --run-after, which has other bugs that make it hard to test, so it's a good interim fix.

Also fixes some whitespace formatting that mixed tabs and spaces.

See https://github.com/mmckegg/msi-packager/issues/19 for more details.