Separate allTargets variables for api.txt code generators.

This change removes the top level variable `allTargets` from
`pkg/analyzer_utilities/lib/tool/api.dart` and replaces it with a
method `allTargetsForPackage`, parameterized by the name of the
package for which an `api.txt` file is generated. The individual code
generators that make use of this function now all have their own top
level variable `allTargets`.

In addition to being conceptually clearer (since we no longer have a
single `allTargets` variable with different meanings depending on
where it's used), this paves the way for a CL I am working on that
will change the `GeneratedContent` based code generators so that they
use paths relative to the `pkg` directory. That in turn should make it
simpler to move some of the code generated files that have to do with
error messages from `pkg/analyzer` to `pkg/_fe_analyzer_shared`.

Change-Id: Id636ddbca27c46ae4242bbf8e4b6f7b8dae4d4e9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/438481
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Auto-Submit: Paul Berry <paulberry@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>