The `sources` key of the project spec only supported directories and not
files. Now it supports both!
This commit introduces a `getSourceFiles` overload that doesn't explicitly
invoke `path.children()`, but instead accepts `children` as a parameter.
This allows us to invoke the `children` overload of getSourceFiles with just
the files we want to include (determined by specifying the sources).
Now for sourcePaths that are files, we group by parents before invoking
getSourceFiles in order to reuse the same groups.
Brandon Kase
Yonas Kolb