[analyzer] Combine FeatureSetProviders getFeatureSet and getLanguageVersion

They are called one after another in `FileSystemState._newFile` and both
call `_findPackage` which is quite expensive.

Running `out/ReleaseX64/dart-sdk/bin/dart analyze --suppress-analytics --format=json --cache=tmpcache1 pkg/analyzer` on a filled cache goes from

```
2.96
2.97
2.97
3.01
2.99
```

to

```
2.86
2.82
2.84
2.84
2.86
```

Statistics on it gives

```
Difference at 95.0% confidence
        -0.136 +/- 0.0268924
        -4.56376% +/- 0.878435%
        (Student's t, pooled s = 0.0184391)
```

Change-Id: I395bea01ebbc7822bf7962587df3227dcca417b0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/541960
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>