fw/plugin: Try to load plugin paths as Python module name

WA_PLUGIN_PATHS currently contains a list of filesystem paths to scan
for plugins. This is appropriate for end-user plugins, but this is
problematic for plugins distributed by a 3rd party, such as a plugin
installed from PyPI.

In those cases, the path to the sources is unknown and typically depends
on the specify Python version, local setup etc. What is constant is
Python name of the package, e.g. "lisa.wa.plugins".

Extend the input allowed in WA_PLUGIN_PATHS by trying to load entries as
a Python package name if:
    * There is no filesystem path with that name
    * The entry is a "relative path" (from an fs point of view)

wa/framework/plugin.py

@@ -622,19 +622,26 @@ def _discover_from_paths(self, paths, ignore_paths):
             self.logger.debug('Checking path %s', path)
             if os.path.isfile(path):
                 self._discover_from_file(path)
-            for root, _, files in os.walk(path, followlinks=True):
-                should_skip = False
-                for igpath in ignore_paths:
-                    if root.startswith(igpath):
-                        should_skip = True
-                        break
-                if should_skip:
-                    continue
-                for fname in files:
-                    if os.path.splitext(fname)[1].lower() != '.py':
+            elif os.path.exists(path):
+                for root, _, files in os.walk(path, followlinks=True):
+                    should_skip = False
+                    for igpath in ignore_paths:
+                        if root.startswith(igpath):
+                            should_skip = True
+                            break
+                    if should_skip:
                         continue
-                    filepath = os.path.join(root, fname)
-                    self._discover_from_file(filepath)
+                    for fname in files:
+                        if os.path.splitext(fname)[1].lower() != '.py':
+                            continue
+                        filepath = os.path.join(root, fname)
+                        self._discover_from_file(filepath)
+            elif not os.path.isabs(path):
+                try:
+                    for module in walk_modules(path):
+                        self._discover_in_module(module)
+                except Exception:
+                    pass
 
     def _discover_from_file(self, filepath):
         try: