Add help message when no arguments are provided

[wagoodman]

Now when no arguments are provided, the default help text is displayed:

$ syft packages
Generate a packaged-based Software Bill Of Materials (SBOM) from container images and filesystems

Usage:
   packages [SOURCE] [flags]

...

an image/directory argument is required
exit status 1

[github-actions]

Benchmark Test Results

Benchmark results from the latest changes vs base branch

name                                                   old time/op    new time/op    delta
ImagePackageCatalogers/ruby-gemspec-cataloger-2           895µs ± 2%     997µs ± 2%  +11.34%  (p=0.008 n=5+5)
ImagePackageCatalogers/python-package-cataloger-2        1.24ms ± 5%    1.30ms ± 4%     ~     (p=0.095 n=5+5)
ImagePackageCatalogers/javascript-package-cataloger-2     457µs ± 3%     493µs ± 3%   +8.01%  (p=0.008 n=5+5)
ImagePackageCatalogers/dpkgdb-cataloger-2                 448µs ± 4%     471µs ± 1%   +5.18%  (p=0.008 n=5+5)
ImagePackageCatalogers/rpmdb-cataloger-2                  485µs ± 2%     501µs ± 2%   +3.15%  (p=0.016 n=5+5)
ImagePackageCatalogers/java-cataloger-2                  6.24ms ± 9%    6.50ms ± 3%     ~     (p=0.151 n=5+5)
ImagePackageCatalogers/apkdb-cataloger-2                  696µs ± 2%     717µs ± 2%   +2.94%  (p=0.032 n=5+5)
ImagePackageCatalogers/go-cataloger-2                     257µs ± 4%     258µs ± 4%     ~     (p=0.841 n=5+5)
ImagePackageCatalogers/rust-cataloger-2                   372µs ± 4%     393µs ± 3%   +5.60%  (p=0.016 n=5+5)

name                                                   old alloc/op   new alloc/op   delta
ImagePackageCatalogers/ruby-gemspec-cataloger-2          97.6kB ± 0%    97.6kB ± 0%     ~     (p=1.000 n=5+5)
ImagePackageCatalogers/python-package-cataloger-2         579kB ± 0%     579kB ± 0%     ~     (p=0.222 n=5+5)
ImagePackageCatalogers/javascript-package-cataloger-2     112kB ± 0%     112kB ± 0%   -0.10%  (p=0.008 n=5+5)
ImagePackageCatalogers/dpkgdb-cataloger-2                 116kB ± 0%     115kB ± 0%     ~     (p=0.222 n=5+5)
ImagePackageCatalogers/rpmdb-cataloger-2                  134kB ± 0%     134kB ± 0%   +0.01%  (p=0.008 n=5+5)
ImagePackageCatalogers/java-cataloger-2                  1.80MB ± 0%    1.79MB ± 0%   -0.21%  (p=0.008 n=5+5)
ImagePackageCatalogers/apkdb-cataloger-2                 1.14MB ± 0%    1.14MB ± 0%     ~     (p=0.817 n=5+5)
ImagePackageCatalogers/go-cataloger-2                    53.9kB ± 0%    53.8kB ± 0%   -0.24%  (p=0.008 n=5+5)
ImagePackageCatalogers/rust-cataloger-2                  88.9kB ± 0%    88.9kB ± 0%   +0.01%  (p=0.016 n=5+4)

name                                                   old allocs/op  new allocs/op  delta
ImagePackageCatalogers/ruby-gemspec-cataloger-2           1.96k ± 0%     1.96k ± 0%     ~     (all equal)
ImagePackageCatalogers/python-package-cataloger-2         5.89k ± 0%     5.89k ± 0%     ~     (all equal)
ImagePackageCatalogers/javascript-package-cataloger-2     1.93k ± 0%     1.93k ± 0%     ~     (all equal)
ImagePackageCatalogers/dpkgdb-cataloger-2                 2.37k ± 0%     2.37k ± 0%     ~     (all equal)
ImagePackageCatalogers/rpmdb-cataloger-2                  3.19k ± 0%     3.19k ± 0%     ~     (all equal)
ImagePackageCatalogers/java-cataloger-2                   22.3k ± 0%     22.3k ± 0%     ~     (all equal)
ImagePackageCatalogers/apkdb-cataloger-2                  1.85k ± 0%     1.85k ± 0%     ~     (p=0.444 n=5+5)
ImagePackageCatalogers/go-cataloger-2                     1.44k ± 0%     1.44k ± 0%     ~     (all equal)
ImagePackageCatalogers/rust-cataloger-2                   2.75k ± 0%     2.75k ± 0%     ~     (all equal)

[luhring]

Looks good! We can probably remove the extra lines of output after Use " [command] --help" ..., but that can be done later. Having the help text back is the high order bit here 🎉