Bump pub to a949b329b1b51f5f3973a790e0a0a45897d837de

Changes:
```
> git log --format="%C(auto) %h %s" a3a102a..a949b32
 https://dart.googlesource.com/pub.git/+/a949b329 Fix handling of 'git@github.com:dart-lang/pub.git' style urls in git source (#3381)
 https://dart.googlesource.com/pub.git/+/c6cd3758 Always use forward slash for relative paths in lockfile (#3379)
 https://dart.googlesource.com/pub.git/+/7d48f902 Remove unused function (#3370)
 https://dart.googlesource.com/pub.git/+/b1373d4c Fix import prefix (#3369)

```

Diff: https://dart.googlesource.com/pub.git/+/a3a102a549388a6dbfecc9252fabb618f9a2f5f7~..a949b329b1b51f5f3973a790e0a0a45897d837de/

A bunch of bugfixes.

Change-Id: I1f791cad27194359091f489f36dcc3ad2b78bb31
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/240052
Reviewed-by: Jonas Jensen <jonasfj@google.com>
Commit-Queue: Sigurd Meldgaard <sigurdm@google.com>

DEPS

@@ -141,7 +141,7 @@ vars = {
   "pool_rev": "7abe634002a1ba8a0928eded086062f1307ccfae",
   "process_rev": "56ece43b53b64c63ae51ec184b76bd5360c28d0b",
   "protobuf_rev": "c1eb6cb51af39ccbaa1a8e19349546586a5c8e31",
-  "pub_rev": "a3a102a549388a6dbfecc9252fabb618f9a2f5f7",
+  "pub_rev": "a949b329b1b51f5f3973a790e0a0a45897d837de",
   "pub_semver_rev": "ea6c54019948dc03042c595ce9413e17aaf7aa38",
   "root_certificates_rev": "692f6d6488af68e0121317a9c2c9eb393eb0ee50",
   "rust_revision": "b7856f695d65a8ebc846754f97d15814bcb1c244",