Rework loading images from files (#7088)

This PR is a follow-up to #7084, where I noted that I wasn't satisfied
with using `SharedUri` to represent both URIs and paths on the local
filesystem:

> I'm still not entirely happy with this naming, as the file paths that
we can store in here are not _really_ URIs, as they are lacking a
protocol.
>
> I want to explore changing `SharedUri` / `SharedUrl` back to alway
storing a URL and treat local filepaths differently, as it seems we're
conflating two different concerns under the same umbrella, at the
moment.

`SharedUri` has now been reverted to just containing a `SharedString`
with a URI.

`ImageSource` now has a new `File` variant that is used to load an image
from a `PathBuf`.

Release Notes:

- N/A

crates/collab/src/tests/integration_tests.rs

@@ -9,7 +9,7 @@ use fs::{repository::GitFileStatus, FakeFs, Fs as _, RemoveOptions};
 use futures::StreamExt as _;
 use gpui::{
     px, size, AppContext, BackgroundExecutor, Model, Modifiers, MouseButton, MouseDownEvent,
-    SharedUri, TestAppContext,
+    TestAppContext,
 };
 use language::{
     language_settings::{AllLanguageSettings, Formatter},
@@ -1828,7 +1828,7 @@ async fn test_active_call_events(
             owner: Arc::new(User {
                 id: client_a.user_id().unwrap(),
                 github_login: "user_a".to_string(),
-                avatar_uri: SharedUri::network("avatar_a"),
+                avatar_uri: "avatar_a".into(),
             }),
             project_id: project_a_id,
             worktree_root_names: vec!["a".to_string()],
@@ -1846,7 +1846,7 @@ async fn test_active_call_events(
             owner: Arc::new(User {
                 id: client_b.user_id().unwrap(),
                 github_login: "user_b".to_string(),
-                avatar_uri: SharedUri::network("avatar_b"),
+                avatar_uri: "avatar_b".into(),
             }),
             project_id: project_b_id,
             worktree_root_names: vec!["b".to_string()]