Corrects scenario where no src_dir is set - 6.x

[cerrussell]

Trying to get the src_dir from the bom path is good, but we need to still set src_dir to the current working directory if depscan hasn't been run using the --bom argument (or when the bom does not actually exist). Otherwise, we will run into problems as we later assume src_dir has been set and try to use it, which will result in an exception.

    if not src_dir or src_dir == ".":
        if src_dir == "." or args.search_purl:
            src_dir = os.getcwd()
        # Try to infer from the bom file
        elif args.bom and os.path.exists(args.bom):
            src_dir = os.path.dirname(os.path.realpath(args.bom))

This PR corrects the problem, as well as a couple other minor improvements to improve quality and maintainability.

[prabhu]

Needs some more fixes I guess.

Traceback (most recent call last):
  File "/home/runner/work/dep-scan/dep-scan/depscan/cli.py", line 1154, in <module>
    main()
  File "/home/runner/work/dep-scan/dep-scan/depscan/cli.py", line 1074, in main
    vdb_results, pkg_aliases, sug_version_dict, purl_aliases = scan(
  File "/home/runner/work/dep-scan/dep-scan/depscan/cli.py", line 356, in scan
    override_results, _, _ = utils.search_pkgs(
  File "/home/runner/work/dep-scan/dep-scan/depscan/lib/utils.py", line 229, in search_pkgs
    variations = normalize.create_pkg_variations(pkg)
  File "/home/runner/work/dep-scan/dep-scan/depscan/lib/normalize.py", line 103, in create_pkg_variations
    if "golang" not in vendor and name not in [
TypeError: argument of type 'NoneType' is not iterable

[cerrussell]

@prabhu Yes, I did not take into account that the vendor key might be present but the value NoneType. Fixed now.

[prabhu]

Take #251 once it is merged to get the CI fix.

[prabhu]

Traceback (most recent call last):
  File "/home/runner/work/dep-scan/dep-scan/depscan/cli.py", line 1154, in <module>
    main()
  File "/home/runner/work/dep-scan/dep-scan/depscan/cli.py", line 1074, in main
    vdb_results, pkg_aliases, sug_version_dict, purl_aliases = scan(
                                                               ^^^^^
  File "/home/runner/work/dep-scan/dep-scan/depscan/cli.py", line 347, in scan
    sug, aliases = process_suggestions(k, v)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/dep-scan/dep-scan/depscan/cli.py", line 401, in process_suggestions
    vendor, name, version = full_pkg.split(":")
    ^^^^^^^^^^^^^^^^^^^^^
ValueError: too many values to unpack (expected 3)
Error: Process completed with exit code 1.