refactor(commands): separate commands and its core into two folders (#32)

- I think it would be better if one for commands only, one for the spec
handling core logic so people won't be confused by the core logic inside
the commands folder (cleaner I guess)
- Regen docs

spec/system.go

@@ -0,0 +1,38 @@
+package spec
+
+import (
+	"os"
+	"path/filepath"
+	"sync"
+)
+
+var (
+	pathCmds     = make(map[string]bool)
+	pathCmdsOnce sync.Once
+)
+
+// scanExternalCommands populates pathCmds with system executables
+func scanExternalCommands() {
+	pathCmdsOnce.Do(func() {
+		scanPath()
+	})
+}
+
+// scanPath populates pathCmds with all executable files found in path environment variable
+func scanPath() {
+	dirs := filepath.SplitList(os.Getenv("PATH"))
+	for _, dir := range dirs {
+		files, err := os.ReadDir(dir)
+		if err != nil {
+			continue
+		}
+		for _, f := range files {
+			if !f.IsDir() {
+				info, err := f.Info()
+				if err == nil && info.Mode()&0111 != 0 {
+					pathCmds[f.Name()] = true
+				}
+			}
+		}
+	}
+}