Update for Bazel 0.25.1

• Minor documentation improvements and code cleanup.
• Satisfy syntax requirements of Bazel 0.25.1

Remove needless debugging output

Honoring bazel dev guidelines:

Do not use the print() function in production code; it is only intended for debugging, and will spam all direct and indirect users of your .bzl file.

Added an optional argument to the generate function so as to only emit debugging output if debug = True.

More resilient copying of vendor to the bazel sandbox

Previous release used cp -r to replicate the vendor directory in the bazel build sandbox. However, -r incorrectly handles malformed symlinks (which are beyond our control). Move to using -a (archive) which will do the right thing.

Note that -a behavior is slightly different on various platforms (macOS is -pPR and GNU tooling is -dR) but these differences shouldn't matter for our use.

Initial Release

Minor tweaks and expanded documentation, otherwise this is just an export of the ActiveState internal build_tools used for Bazelizing a standard golang vendor dependencies directory tree.