Clarify internal validation logic + don't check for change needed in bump command

[ecraig12345]

Previously validate() had an option allowFetching (default true) which was hiding the actual behavior. It was also not precise about which commands needed to validate dependencies or not. Update the options as follows:

• checkChangeNeeded: check whether change files are needed (change, check)
• allowMissingChangeFiles: used with above, but don't error on missing change files (change)
• checkDependencies: perform an in-memory bump and validate that no package to be bumped (including dependents) have private dependencies (most commands but change). Previously in some cases, change would include this check, which just slows it down and isn't needed.

As a side effect of the confusing options, previously the bump command was checking for whether change files are needed, which was odd and probably an artifact of allowFetching defaulting to true, not intentional.