build.py doesn't copy to subdirs of the target directory

[aaaaalbert]

SeattleTestbed/dist's build.py (aka "the new build script") currently lacks the functionality to copy files to subdirectories of the build target dir. Similar to #140, the old build harness allows this, e.g. to create subfolders for the different Repy runtimes in a Seattle installer.

I think we could add the subdir as an optional second or third argument in config_build.txt, like

# This one would be for building a component
seattlelib_v2/* subdir_for_seattlelib

# And that's the syntax for preparing tests
test repy_v2/* subdir_for_repytests

[aaaaalbert]

build.py turned into build_component.py and now lives in the SeattleTestbed/common repo. Closing this ticket, and reopening it as SeattleTestbed/common#8.