Problem: Installation confirmation has several usability issues:
- Choosing "No" results in a `vim.pack.add()` error. This was by
design to ensure that all later code that *might* reference
presumably installed plugin will not get executed. However, this
is often too restrictive since there might be no such code (like
if plugin's effects are automated in its 'plugin/' directory).
Instead the potential code using not installed plugin will throw
an error.
No error on "No" will also be useful for planned lockfile repair.
- List of soon-to-be-installed plugins doesn't mention plugin names.
This might be confusing if plugins are installed under different
name.
Solution: Silently drop installation step if user chose "No" and show
plugin names in confirmation text (together with their pretty aligned
sources).
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