Refactor dep processing to improve efficiency of installation

Assemble single list of dependencies to reduce duplication and reduce installation overhead

tox_poetry_installer/exceptions.py

@@ -5,6 +5,7 @@ All exceptions should inherit from the common base exception :exc:`ToxPoetryInst
 ::
 
   ToxPoetryInstallerException
+   +-- SkipEnvironment
    +-- LockedDepVersionConflictError
    +-- LockedDepNotFoundError
    +-- ExtraNotFoundError
@@ -17,6 +18,10 @@ class ToxPoetryInstallerException(Exception):
     """Error while installing locked dependencies to the test environment"""
 
 
+class SkipEnvironment(ToxPoetryInstallerException):
+    """Current environment does not meet preconditions and should be skipped by the plugin"""
+
+
 class LockedDepVersionConflictError(ToxPoetryInstallerException):
     """Locked dependencies cannot specify an alternate version for installation"""