GeezerVison

`install` appended a `MetadataPathFinder` to `sys.meta_path` while the
module body was still executing, 36 lines before `PathDistribution` is
bound, and `PathDistribution` is the only global the installed finder has
yet to see (`Prepared`, `FastPath` and `DistributionFinder` all precede
it). Anything walking `sys.meta_path` in that window resolves it against
incomplete globals and raises `NameError: name 'PathDistribution' is not
defined`.

Two conditions reach the window. Another thread calling `entry_points()`
takes no module lock, as it imports nothing and only iterates
`sys.meta_path`. And an import aborted in the window, as an async
exception such as a signal handler or a soft time limit can do, leaves
the half-built finder installed for the life of the process: the module
is dropped from `sys.modules`, `install` has already deleted
`PathFinder.find_distributions` so the orphan is the only provider, and
even a later successful import does not displace it.

Installing at the end of the module, rather than just after
`PathDistribution`, states the invariant the finder depends on and so
survives any later reordering.

The stdlib copy is unaffected, where `install` is stripped and
`PathFinder.find_distributions` imports `MetadataPathFinder` lazily.

Closes #525.

Co-Authored-By: Claude Opus 5 <[email protected]>

The comment explaining why the call sits at the end of the module was
four more lines for the CPython port to strip. `install` is itself
removed there, so its docstring carries the constraint for free.

Co-Authored-By: Claude Opus 5 <[email protected]>

`install` has no counterpart in the stdlib, which reaches
`MetadataPathFinder` through its own `PathFinder` and publishes no finder
of its own, so the expectation belongs beside the other backport-specific
tests rather than among those test_main holds for both codebases.

Co-Authored-By: Claude Opus 5 <[email protected]>

Nothing consumes the return value now that the call is a statement, and
withholding it makes the decorator form bind None rather than quietly
restoring the window it was moved to close.

Co-Authored-By: Claude Opus 5 <[email protected]>