Problem: If plugin was intended to install but there were errors (like
if there is a typo in `src`), lockfile still includes its entry.
This leads to all source of problems (like not correct `get()` output,
not working `update()`, etc.).
Solution: Explicitly account for plugins that were not installed.
Alternative solution might be to write a safe
`lock_set(plug, field, value)` wrapper (which sets field for a correct
`plugins` entry in the lockfile Lua table) and use it after install
to detect the change in `version`. However, this always requires
an extra pass through plugins on every startup, which is suboptimal.
Optimizing for the "happy path" should be a priority in `add()`.
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