update PBS generate_urls.py housekeeping script with resolve and options

[tdyas]

Some small updates to the PBS generate_urls.py housekeeping script:

• Put the script into a pbs-script resolve to isolate its dependencies.
• Add a pex_binary target for running the script.
• Use argparse to add new options --scrape-all-releases and --scrape-release=RELEASE for forcing the script to re-process already-scraped releases. I am using these for another PR where I modified the data format for versions_info.json and need to rescrape the PBS release data.
• The build root check is now just a check that the script is being run from the root of a Pants repository (via presence of the expected subdirectories). (This avoids needing to import get_buildroot from the other Pants sources.)