conservative locking can raise keyerror in some circumstances

[stefanoborini]

When invoking roo lock --conservative, I managed to get this error.

(bootstrap-venv:r-library-maic)$ roo lock --conservative
Traceback (most recent call last):
File "bootstrap-venv/bin/roo", line 8, in
sys.exit(main())
File "bootstrap-venv/lib/python3.9/site-packages/click/core.py", line 829, in call
return self.main(*args, **kwargs)
File "bootstrap-venv/lib/python3.9/site-packages/click/core.py", line 782, in main
rv = self.invoke(ctx)
File "bootstrap-venv/lib/python3.9/site-packages/click/core.py", line 1259, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "bootstrap-venv/lib/python3.9/site-packages/click/core.py", line 1066, in invoke
return ctx.invoke(self.callback, **ctx.params)
File "bootstrap-venv/lib/python3.9/site-packages/click/core.py", line 610, in invoke
return callback(*args, **kwargs)
File "bootstrap-venv/lib/python3.9/site-packages/roo/cli/lock/init.py", line 29, in lock
_ensure_lock(overwrite, conservative)
File "bootstrap-venv/lib/python3.9/site-packages/roo/cli/lock/init.py", line 61, in _ensure_lock
new_lock = locker.lock(rproject, old_lock, conservative)
File "bootstrap-venv/lib/python3.9/site-packages/roo/locker.py", line 71, in lock
resolver.resolve_full_tree(root, old_lock_tree)
File "bootstrap-venv/lib/python3.9/site-packages/roo/resolver.py", line 54, in resolve_full_tree
self._pre_populate_cache(root, old_tree)
File "bootstrap-venv/lib/python3.9/site-packages/roo/resolver.py", line 81, in _pre_populate_cache
del self.resolved_cache[dep.name]
KeyError: 'knitr'