Pyenv will install shims for commands that exist in any interpreter, even if it is not the current one. This means that a command may technically exist, but when executed will tell the user to try a different interpreter. The original check for pip in the Python module can fail for this reason, in particular on Ubuntu 20.04.
This change checks with pyenv whether pip really exists in the current interpreter to work around this problem and fixes a bug in pip command detection.