This simplifies the handling of the subdirectories in the SIL and
SILOptimizer paths. Create individual libraries as object libraries
which allows the analysis of the source changes to be limited in scope.
Because these are object libraries, this has 0 overhead compared to the
previous implementation. However, string operations over the filenames
are avoided. The cost for this is that any new sub-library needs to be
added into the list rather than added with the special local function.
Move the source files from ParserSIL into the SIL library and remove the
ParserSIL library. ParsersSIL doesn't need to be its own library and this change will
simplify our builds.