chore(developer): only build source files

For batch builds (e.g. project/compile all) of v2.0 projects, we only
want to build files that are listed in the project's $SOURCEPATH, so
that the project can contain extra folders with source file types that
won't break a project-level build.

Any source file types in extra folders can still be built one-by-one,
which is helpful for investigation and test for keyboard authors, but
will not be built if project build is selected.

This commit also formalizes the source/ SourcePath for upgraded projects
which contain a source/ folder. If the project to be upgraded does not
contain the source/ folder, then no SourcePath is assigned, and any
source file in any folder will be built.