Replace `tool/update-deps` with a Ruby source scanner so dependency
updates no longer require GNU make, a configured build, or compiler
preprocessor output.
Keep compact source mappings and scanner declarations in dependency
files, and expand complete rules into the build directory. Avoid the
temporary fixture headers required by the compiler-driven prototype.
Use the generated rules from GNU make, BSD make, and NMake builds.
Support out-of-tree and read-only source trees, and retain expanded
dependencies shipped in release archives when baseruby is unavailable.
Nobuyoshi Nakada