Refactor: use shutil.which instead of manual PATH walk in PosixHelpRenderer

## Summary

\`PosixHelpRenderer._exists_on_path\` in \`awscli/help.py\` (lines ~162-170) manually reimplements PATH lookup by joining each PATH entry with \`os.path.join\`/\`os.path.exists\` inside a list comprehension, then calls \`any()\` on the resulting list.

## Proposed change

Use the stdlib \`shutil.which\` (available since Python 3.3): \`return shutil.which(name) is not None\`. Removes the manual PATH-walking logic and avoids building a throwaway list. Pure refactor, no behavior change.

Start by reviewing PosixHelpRenderer._exists_on_path and the surrounding imports in awscli/help.py, around lines 162-170. Done means the manual PATH walk is replaced as proposed without changing the method's behavior.

Written by the indexing model from the issue text.