Helper script to copy source files to the import directory

When building druntime, files in src are copied to the import directory, excluding modules from the rt and gc packages and a couple more non-public files. On POSIX this is handled by a Makefile rule, however DigitalMars' make on Windows has very limited support for custom rules, originally leading to an unhealthy duplication between mak/COPY and mak/WINDOWS, which this script removes.