Enhance command_exists? method so that it falls to which in case the command fails

[pingport80]

Summary

This PR adds a small enhancement to the command_exists? method under post/common.rb. Currently it is using the command command to check if some command is present on the target system but in some cases that utility might not be present(see here for an example). So in that case it will fall to some other alternative like which.

This PR also removes a method pids which was removed in the PR #15199 but while refactoring it's code in the new process.rb the change was missed.

[wvu]

/me laughs in POSIX.

[smcintyre-r7]

Totally out of scope for this PR, but at some point, we may want to consider some kind of caching to determine what commands are available on a remote system since it's unlikely to change (unless we're changing it I guess) and each query takes a round trip which slows things down.

[pingport80]

That really sounds interesting and will be very useful. I guess for non-windows we can get using the dir method in every directory the PATH returns. For Powershell something like Get-Command, not sure about windows cmd.

[gwillcox-r7]

LGTM, will land this now.

[gwillcox-r7]

Release Notes

The command_exists? method inside post/common.rb has been updated to fall back to using the which command to check if a command exists on a target system if command -v fails to run successfully. This allows users to check whether a command exists or not on systems that might not contain a command command, such as ESXi.