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
resolution	ResolutionTooDeepError
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 resolution.

Tip

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