implements a fixed-length build prefix

This pull request implements a fixed-length build path for recipes with binary_has_prefix_files. The length needs to be fixed so packages can be successfully installed into another developers build environment. The build path is currently padded to 80 characters, but this could be increased. There were concerns about build paths that are too long for hashbang lines on unix os.

This pull request also attempts to simplify the organization and logic associated with dealing short or long build paths and selection between the two.