Consider getting rid of package:analyzer_cli

[stereotype441]

In an internal doc where I talked about package:analyzer_cli, @bwilkerson suggested:

We should consider getting rid of this [package]. The only reason it's currently needed (that I'm aware of) is because dart analyze doesn't support the "read files to be analyzed from stdin" feature.

Note that package:analyzer_cli is a small utility library used by test.py to run automated tests (such as language tests and co19 tests) through package:analyzer. It’s not the command-line interface of the analyzer exposed to users through dart analyze.