Feature request: auto-detect when update_packages() requires standalone mode

[pnacht]

As discussed in #613, update_packages must be in standalone mode to update its own dependencies such as curl, etc.

However, this implies that if one wants to update packages, one must:

• run update_packages()
• have the relevant dependencies memorized
• check for those dependencies in the list of updateable packages
• if any are found, cancel the update
• set the Standalone Mode environment variable
• run update_packages() again

(I assume there's a reason to prefer using those dependencies whenever possible, so simply setting Standalone Mode every time doesn't seem reasonable)

Would it be possible to perform the check within update_packages? That is: fetch the list of updateable packages (or maybe the subset requested by the user), see if any of the relevant dependencies are on that list, and set Standalone Mode as needed?