refactor: Split exceptions into a package

[ichard26]

I'm planning to add even more diagnostic errors, but exceptions.py is starting to become unwieldy. I've converted it into a package to permit for further growth. To start, I've moved the following exceptions to their own submodules.

Group	Exceptions moved
pyproject	MissingPyProjectBuildRequires, InvalidPyProjectBuildRequires
wheel	InvalidWheelFilename, UnsupportedWheel, InvalidWheel
hashes	HashErrors, HashError, VcsHashUnsupported, DirectoryUrlHashUnsupported, HashMissing, HashUnpinned, HashMismatch
uninstall	UninstallMissingRecord, LegacyDistutilsInstall
build_env	BuildDependencyInstallError, VenvImportError, VenvCreationError
network	NetworkConnectionError, ConnectionFailedError, ConnectionTimeoutError, SSLMissingError, SSLVerificationError, ProxyConnectionError, IncompleteDownloadError

I've left the remaining exceptions alone in exceptions/__init__.py. I'm happy to hear suggestions for more groupings.

In the near-to-medium future, I'm planning to add new diagnostic errors to wheel (and possibly a new resolution group).

Tip

This is probably much easier to review commit-by-commit.

[ichard26]

I've rebased this onto current main and massively simplified the diff by re-exporting the moved exceptions in exceptions/__init__.py to avoid impacting imports in ~30 pip source files.

Since this is going to conflict with the other error improvement work I'm doing, I'd like to get this landed sooner than later. Unless someone objects, I'll merge this within a few days.