Update SemIR yaml and text for multi-file. (#3217)

Building on #3214 and #3215, updates sem_ir yaml to be:

```
- filename: name
  sem_ir: [ ... ]
```

Also, changes the textual format from `package { ... }` to `file
<filename> { ... }`. My thought on packages there is:

```
file "foo.carbon" {
  package MyPackage

  ...
}
```

The reason for putting the file first is that it's easier if we put what
we're grouping on first, whereas the package is an "annotation" on the
file.

toolchain/check/testdata/function/declaration/simple.carbon

@@ -8,7 +8,7 @@ fn F();
 
 fn G() { F(); }
 
-// CHECK:STDOUT: package {
+// CHECK:STDOUT: file "simple.carbon" {
 // CHECK:STDOUT:   %.loc7 = fn_decl @F
 // CHECK:STDOUT:   %.loc9 = fn_decl @G
 // CHECK:STDOUT: }