This centralizes verification and common operations, like converting the
path to a source file to the path for a built object.
It also embeds the configuration knowledge into the path, so that we can
remove "${SrcDir}/path" from the ninja file. When SrcDir is '.', that
leads to paths like './path' instead of just 'path' like make is doing,
causing differences in compiled binaries.
Change-Id: Ib4e8910a6e867ce1b7b420d927c04f1142a7589e
