Add a new entry_point for xtriggers

This creates a clean way to add xtriggers from python packages.
xtriggers no longer need to be directly in the PYTHONPATH.

cylc/flow/scripts/install.py

@@ -343,6 +343,20 @@ def install(
                 exc
             ) from None
 
+    for entry_point in iter_entry_points(
+        'cylc.xtriggers'
+    ):
+        try:
+            entry_point.resolve()()
+        except Exception as exc:
+            # NOTE: except Exception (purposefully vague)
+            # this is to separate plugin from core Cylc errors
+            raise PluginError(
+                'cylc.xtriggers',
+                entry_point.name,
+                exc
+            ) from None
+
     print(f'INSTALLED {workflow_id} from {source_dir}')
 
     return workflow_name, workflow_id

cylc/flow/subprocpool.py

@@ -43,6 +43,12 @@
 
 _XTRIG_FUNCS: dict = {}
 
+# Add in xtriggers from entry_points for external sources
+for entry_point in iter_entry_points('cylc.xtriggers'):
+    entry = entry_point.resolve()()
+    for func_name, func in entry.items():
+        _XTRIG_FUNCS[func_name] = func
+
 
 def _killpg(proc, signal):
     """Kill a process group."""