Many moz.build files define things that could be defined in parent
moz.build files. Keeping the number of moz.build files low helps with
build times due to less I/O and fewer directories traversed.
This patch eliminates a lot of moz.build files by moving their content
into parent moz.build files.
Every directory with a jar.mn now has JAR_MANIFESTS defined in its
moz.build file.
We also removed the may_skip special consideration of jar.mn files
because this information is now available during tier traversal by the
reader courtesy of the variables being present in moz.build files.
