fix(script): Tweak cargo script build-dir / target-dir (#16086)

### What does this PR try to resolve?

This is meant to unblock efforts for allowing cargo scripts to use
artifact names like `deps` without being sanitized or erroring. This is
done by making the default `target-dir` exclude all `build-dir` content
by putting the default cargo script `target-dir` under `build-dir`.

Cargo script target-dir precedence
| before | after|
|-|-|
| explicit `target-dir`| explicit `target-dir`|
| `"{cargo-cache-home}/target/{workspace-path-hash}"` |
**`"{build-dir}/target"`** |

Cargo script build-dir precedence
| before | after|
|-|-|
| explicit `build-dir`| explicit `build-dir`|
| explicit `target-dir`| explicit `target-dir`|
| `"{cargo-cache-home}/target/{workspace-path-hash}"` |
`"{cargo-cache-home}/target/{workspace-path-hash}"` |

In doing this split, it highlighted the fact that we still had
`Cargo.lock` for cargo scripts in `target-dir` despite being an
intermediate build artifact, so this also moves that into `build-dir`.
This does mean that if someone has a shared `build-dir` between their
projects, their scripts will get a bit confusing. I plan to highlight
that in the tracking issue / stabilization report.

### How to test and review this PR?

## Notes

Work left to do for artifact names
- Stop creating `examples/` when its not used
- Move the conflict check from parse time to build time
- Make the conflict check dependent on if `target_dir == build_dir`

Author: weihanglo

src/cargo/core/workspace.rs

@@ -439,21 +439,32 @@ impl<'gctx> Workspace<'gctx> {
     }
 
     pub fn build_dir(&self) -> Filesystem {
-        self.build_dir.clone().unwrap_or_else(|| self.target_dir())
+        self.build_dir
+            .clone()
+            .or_else(|| self.target_dir.clone())
+            .unwrap_or_else(|| self.default_build_dir())
     }
 
     fn default_target_dir(&self) -> Filesystem {
+        if self.root_maybe().is_embedded() {
+            self.build_dir().join("target")
+        } else {
+            Filesystem::new(self.root().join("target"))
+        }
+    }
+
+    fn default_build_dir(&self) -> Filesystem {
         if self.root_maybe().is_embedded() {
             let default = ConfigRelativePath::new(
-                "{cargo-cache-home}/target/{workspace-path-hash}"
+                "{cargo-cache-home}/build/{workspace-path-hash}"
                     .to_owned()
                     .into(),
             );
             self.gctx()
                 .custom_build_dir(&default, self.root_manifest())
                 .expect("template is correct")
         } else {
-            Filesystem::new(self.root().join("target"))
+            self.default_target_dir()
         }
     }