Bump pub to ac7db6c07318efa4a8712110275eaf70f96a6d00

Changes:
```
> git log --format="%C(auto) %h %s" 9bf4289..ac7db6c
 https://dart.googlesource.com/pub.git/+/ac7db6c0 Fix string shortening (#3512)
 https://dart.googlesource.com/pub.git/+/4ed0d5cd Add pubspec_overrides.yaml to basicIgnoreRules (#3503)
 https://dart.googlesource.com/pub.git/+/d8f4c549 Remove all support for legacy .packages file (#3507)
 https://dart.googlesource.com/pub.git/+/1722b6a8 Update 2-BUG.md (#3495)
 https://dart.googlesource.com/pub.git/+/e60ef647 Add lint avoid_print (#3475)

```

Diff: https://dart.googlesource.com/pub.git/+/9bf4289d6fd5d6872a8929d6312bbd7098f3ea9c~..ac7db6c07318efa4a8712110275eaf70f96a6d00/
Change-Id: I730a048dbf10992270f74ddde47824cc071a3994
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/253663
Commit-Queue: Sigurd Meldgaard <sigurdm@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>

CHANGELOG.md

@@ -55,6 +55,11 @@ Updated the Linter to `1.26.0`, which includes changes that
  non-final variable.
 - fix`use_build_context_synchronously` to handle `await`s in `if` conditions.
 
+#### Pub
+
+- Remove remaining support for `.packages` files. The flag
+ `--legacy-packages-file` is no longer supported.
+
 ### Core libraries
 
 #### `dart:io`

DEPS

@@ -136,7 +136,7 @@ vars = {
  "ply_rev": "604b32590ffad5cbb82e4afef1d305512d06ae93",
  "pool_rev": "fa84ddd0e39f45bf3f09dcc5d6b9fbdda7820fef",
  "protobuf_rev": "504eefeae9892602ea50c20159db1db3768012a9",
- "pub_rev": "9bf4289d6fd5d6872a8929d6312bbd7098f3ea9c", # manually rev'd
+ "pub_rev": "ac7db6c07318efa4a8712110275eaf70f96a6d00", # manually rev'd
  "pub_semver_rev": "9fd28757ba45961ac5449e0f2b0020670e921475",
  "root_certificates_rev": "692f6d6488af68e0121317a9c2c9eb393eb0ee50",
  "shelf_rev": "0371a64bd3b99044ee3158bacf8813bba735a9c7",