Avoid duplicate dependency installation

The previous commit changed the resolver logic to try all resolvers, so that
when building an environment fails, the next resolver can be tried. Since
dependency resolution and installation are coupled it would be possible to
install dependencies multiple times for different resolvers (e.g for Conda and
Conda versionless).
To avoid this we stop installation once a set of dependencies has been
resolved.

lib/galaxy/tools/deps/__init__.py

@@ -159,6 +159,9 @@ class DependencyManager(object):
 
                     # Shortcut - resolution complete for this resolver.
                     resolver_requirements.append(requirement_to_dependency)
+                    # We have a complete set of dependencies, don't install
+                    # via lower-ranked resolvers
+                    kwds['install'] = False
                     continue
 
             # Check individual requirements
@@ -178,6 +181,11 @@ class DependencyManager(object):
                     requirement_to_dependency[requirement] = dependency
             if requirement_to_dependency:
                 resolver_requirements.append(requirement_to_dependency)
+                if (len(requirement_to_dependency) == len(resolvable_requirements) and
+                        all(True for d in requirement_to_dependency if not isinstance(d, NullDependency))):
+                    # We resolved all individual requirements, no need to further install dependencies
+                    # resolved by subsequent resolvers.
+                    kwds['install'] = False
         if not resolver_requirements:
             # Have at least a single empty dict
             resolver_requirements.append(OrderedDict())