Don't advertize prerelease packages in `pub get`

[mit-mit]

Running pub get on just about any project now shows the new prerelease nullsafety packages, e.g.:

Resolving dependencies... (2.6s)
+ _fe_analyzer_shared 5.0.0
+ analyzer 0.39.12
+ args 1.6.0
+ async 2.4.2
+ benchmark_harness 1.0.5
+ boolean_selector 2.0.0
+ charcode 1.1.3
+ collection 1.14.13 (1.15.0-nnbd available)
+ convert 2.1.1
+ coverage 0.14.0
+ crypto 2.1.5
...

We should prune the "available" messages to not show prerelease versions

[mit-mit]

Potential fix: Remove the if else from https://github.com/dart-lang/pub/blob/master/lib/src/solver/report.dart#L253 ?

[jonasfj]

Fixed in #2556