`allow-newer` in project file `package` stanza

While updating my projects for, e.g. a new compiler, I often find it necessary to vendorize libraries while waiting for upstream to merge bound bump changes. This is a fair amount of work, but I find it more satisfying than the alternative of just accumulating entries in the global `allow-newer` list.

What I would really like is a way to easily lift a given bound in particular package from within a project file. For instance,
```
package safecopy
allow-newer: vector
```

Start by reading how the project-file package stanza is parsed and how the global allow-newer list is applied. Trace dependency bound handling for the requested package-scoped form, then verify that a project can lift a bound for one named package without changing the global setting.

Written by the indexing model from the issue text.