Add internally proxied poetry module to support runtime in non-Poetry envs

Add the _poetry submodule to support importing Poetry internals at runtime rather
than import time. This allows the plugin to be run, and either skipped or errored,
without crashing tox

tox_poetry_installer/exceptions.py

@@ -6,6 +6,7 @@ All exceptions should inherit from the common base exception :exc:`ToxPoetryInst
 
   ToxPoetryInstallerException
    +-- SkipEnvironment
+   |    +-- PoetryNotInstalledError
    +-- LockedDepVersionConflictError
    +-- LockedDepNotFoundError
    +-- ExtraNotFoundError
@@ -22,6 +23,10 @@ class SkipEnvironment(ToxPoetryInstallerException):
     """Current environment does not meet preconditions and should be skipped by the plugin"""
 
 
+class PoetryNotInstalledError(SkipEnvironment):
+    """No version of Poetry could be imported from the current Python environment"""
+
+
 class LockedDepVersionConflictError(ToxPoetryInstallerException):
     """Locked dependencies cannot specify an alternate version for installation"""