Add 'where' command to locate packages in manifest files

[andrew]

Find where a package is declared

git pkgs where rails           # find in manifest files
git pkgs where lodash -C 2     # show 2 lines of context
git pkgs where express --ecosystem=npm

Shows which manifest files declare a package and the exact line:

Gemfile:5:gem "rails", "~> 7.0"
Gemfile.lock:142:    rails (7.0.8)

Like grep but scoped to manifest files that git-pkgs knows about.