Restores some Composer 1.x behavior like unbound constraints conflicting
with default branches unless they are branch aliased.
Simplifies conflicts with aliases because packages cannot be installed
without their aliases, so we do not need to know which aliases are
uninstalled in lock file or installed.json.
The alias still ends up being marked as installed as the install step
reads it from the branch alias in the lock file and doesn't know a
conflict required it to be skipped.
In complex scenarios reasons for learned rules can themselves be learned
rules caused by other learned rules which had the some of the same
reasons. In this situation iterating over all problem rules requires
keeping track of which rules have previously been analyzed to avoid and
endless loop.
Side effect is that the sorting of problems including learned rules
changes slightly.
Locked packages are basically like removable fixed packages, so we still
only load one version, but we do not require their installation unless
something the user needs requires their use. So they automatically get
removed if they are no longer needed on any update.
Instead of marking locked packages as fixed, we change the pool builder
to load only the locked version and treat it like a fixed package, but
removing the actual request fix, makes the solver treat it as a regular
optional dependency. As a consequence locked packages may be removed on
a partial update of another package, but they cannot be updated.
So far the ordering of alias packages with the same name was undefined
so the actual order was determined by implementation of the sorting
algorithm in PHP. As of PHP8 sort is stable by default which changes the
outcome in some of our test cases.
With the fully defined total order the order can longer change depending
on sorting algorithm used and remains the same across PHP versions.