[Core] Add multiple/external cog paths support (#853)

* WIP cog path manager

* Initial working state

* Get real reloading working

* Type error thingy

* And fix the tests

* Start UI shit

* path reordering

* Add install path getter/setter and fix config syntax

* Determine bot directory at runtime

* Add UI commands for install path

* Update downloader to use install path

* Add sane install path default

* Make evaluation of cog install path lazy

* Some typing fixes

* Add another line to codeowners

* Conditionally put install path in paths

* Always put install path first

* Dont allow people to add the installdir as an additional path, guarantee install dir isn't shown with paths command

* Make shit update loaded cogs

* Add tests

* Another one

tests/core/test_cog_manager.py

@@ -0,0 +1,57 @@
+from pathlib import Path
+
+import pytest
+from core import cog_manager
+
+
+@pytest.fixture()
+def cog_mgr(red):
+    return red.cog_mgr
+
+
+@pytest.fixture()
+def default_dir(red):
+    return red.main_dir
+
+
+def test_ensure_cogs_in_paths(cog_mgr, default_dir):
+    cogs_dir = default_dir / 'cogs'
+    assert cogs_dir in cog_mgr.paths
+
+
+@pytest.mark.asyncio
+async def test_install_path_set(cog_mgr: cog_manager.CogManager, tmpdir):
+    path = Path(str(tmpdir))
+    await cog_mgr.set_install_path(path)
+    assert cog_mgr.install_path == path
+
+
+@pytest.mark.asyncio
+async def test_install_path_set_bad(cog_mgr):
+    path = Path('something')
+
+    with pytest.raises(ValueError):
+        await cog_mgr.set_install_path(path)
+
+
+@pytest.mark.asyncio
+async def test_add_path(cog_mgr, tmpdir):
+    path = Path(str(tmpdir))
+    await cog_mgr.add_path(path)
+    assert path in cog_mgr.paths
+
+
+@pytest.mark.asyncio
+async def test_add_path_already_install_path(cog_mgr, tmpdir):
+    path = Path(str(tmpdir))
+    await cog_mgr.set_install_path(path)
+    with pytest.raises(ValueError):
+        await cog_mgr.add_path(path)
+
+
+@pytest.mark.asyncio
+async def test_remove_path(cog_mgr, tmpdir):
+    path = Path(str(tmpdir))
+    await cog_mgr.add_path(path)
+    await cog_mgr.remove_path(path)
+    assert path not in cog_mgr.paths