Problem: During initial "bootstrap" via lockfile synchronization, the
whole plugin specification is reconstructed from the lockfile data,
ignoring potential user changes added in the first `vim.pack.add()`.
This is enough in most situations since it is the only data needed
for actual installation.
However, this affects specification passed to `PackChanged[Pre]`
events. In particular, `data` field is missing which can be a problem
if there is a `PackChanged kind=install` hook that uses that field
(like with some kind of `build` method used during install).
And there might be different `version` set in `vim.pack.add()`.
Solution: Pass the `specs` input of the first `vim.pack.add()` down to
lockfile synchronization and use it to reconstruct plugin
specification for the to-be-installed plugin. If present among the
user's `specs`, it is used but with forced `src` from the lockfile (as
it is the one used during installation).
Note that this still has a caveat when using separate
`vim.pack.add()`, as only the specs from the first input (when the
lockfile synchronization happens) is taken into account.