Extend comma support to all list values

docs/cli_interface.rst

@@ -71,21 +71,18 @@ variable ``VIRTUALENV_PYTHON`` like:
 
    env VIRTUALENV_PYTHON=/opt/python-3.8/bin/python virtualenv
 
-Multiple values can be provided separated with a comma:
+Where the option accepts multiple values, for example for :option:`python` or
+:option:`extra-search-dir`, the values can be separated either by comma or a
+literal newline:
 
 .. code-block:: console
 
    env VIRTUALENV_PYTHON=/opt/python-3.8/bin/python,python3.8 virtualenv
+   env VIRTUALENV_EXTRA_SEARCH_DIR=/path/to/dists\n/path/to/other/dists virtualenv
 
-This also works for appending command line options, like :option:`extra-search-dir`, where a literal newline
-is used to separate the values:
-
-.. code-block:: console
-
-  env VIRTUALENV_EXTRA_SEARCH_DIR=/path/to/dists\n/path/to/other/dists virtualenv
-
-The equivalent CLI-flags based invocation, for the above example, would be:
+The equivalent CLI-flags based invocation for the above examples would be:
 
 .. code-block:: console
 
+   virtualenv --python=/opt/python-3.8/bin/python --python=python3.8
    virtualenv --extra-search-dir=/path/to/dists --extra-search-dir=/path/to/other/dists

src/virtualenv/config/convert.py

@@ -57,19 +57,22 @@ def convert(self, value, flatten=True):
     def split_values(self, value):
         """Split the provided value into a list.
 
-        For strings this is a comma-separated. For more complex types (`Path`
-        for example) this is newline-separated.
+        First this is done by newlines. If there were no newlines in the text,
+        then we next try to split by comma.
         """
         if isinstance(value, (str, bytes)):
-            if self.as_type is str:
-                # Simple split
-                value = value.split(",")
-            else:
-                value = value.splitlines()
-
-            value = filter(None, [x.strip() for x in value])
-
-        return list(value)
+            # Use `splitlines` rather than a custom check for whether there is
+            # more than one lines. This ensures that the full `splitlines()`
+            # logic is supported here.
+            values = value.splitlines()
+            if len(values) <= 1:
+                values = value.split(",")
+
+            values = filter(None, [x.strip() for x in values])
+        else:
+            values = list(value)
+
+        return values
 
 
 def convert(value, as_type, source):