Add intelligent package completion to yarn

Now parses package.json and uses results to provide a list of possible
completions to `yarn remove`. There may be other subcommands that could
benefit from this.

Could have parsed yarn output, but yarn is slow and packages.json format
is generally standard since it's machine-generated json.

CHANGELOG.md

@@ -56,6 +56,7 @@ This section is for changes merged to the `major` branch that are not also merge
   - `git` (#4395, #4396, #4592)
   - `brew`
   - `diskutil`
+  - `yarn`
 
 --
 

share/completions/yarn.fish

@@ -2,6 +2,54 @@
 # see https://github.com/fish-shell/fish-shell/blob/master/share/functions/__fish_seen_subcommand_from.fish
 # and https://github.com/fish-shell/fish-shell/blob/master/share/functions/__fish_use_subcommand.fish
 
+function __yarn_find_package_json
+    set parents (__fish_parent_directories (pwd))
+
+    for p in $parents
+        if test -f "$p/package.json"
+            echo "$p/package.json"
+            return 0
+        end
+    end
+
+    return 1
+end
+
+function __yarn_list_packages
+    set -l package_json (__yarn_find_package_json)
+    if not test $status -eq 0
+        # no package.json in tree
+        return 1
+    end
+
+    set -l depsFound 0
+    for line in (cat $package_json)
+        # echo "evaluating $line"
+        if test $depsFound -eq 0
+            # echo "mode: noDeps"
+            if string match -qr '(devD|d)ependencies"' -- $line
+                # echo "switching to mode: deps"
+                set depsFound 1
+                continue
+            end
+            continue
+        end
+
+        if string match -qr '\}' -- $line
+            # echo "switching to mode: noDeps"
+            set depsFound 0
+            continue
+        end
+
+        # echo "mode: deps"
+
+        string replace -r '^\s*"([^"]+)".*' '$1' -- $line
+    end
+end
+
+
+complete -f -c yarn -n '__fish_seen_subcommand_from remove' -a (set -l packages (__yarn_list_packages); echo $packages)
+
 complete -f -c yarn -n '__fish_use_subcommand' -a help
 
 complete -f -c yarn -n '__fish_use_subcommand' -a access