[pm] Sort the list of blobs in package_manifest.json.

Emitting the blobs in sorted order makes the contents of
package_manifest.json deterministic for a given set of
inputs. Determinism allows the build to skip steps that
depend on intermediate files unaffected by a change,
even if those files were re-generated.

Bug: 61368
Change-Id: I52dd768f586ae03eb42c8df405d2d07ad560faa3
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/436574
Testability-Review: Kevin Wells <kevinwells@google.com>
Commit-Queue: Wez . <wez@google.com>
Reviewed-by: Clayton Wilkinson <wilkinsonclay@google.com>
Reviewed-by: George Kulakowski <kulakowski@google.com>
Reviewed-by: James Tucker <raggi@google.com>
Reviewed-by: Kevin Wells <kevinwells@google.com>