Require LICENSE file

[jonasfj]

We should:

• analyze how many packages does not contain a LICENSE file.
• require the license file is LICENSE.

[FMorschel]

Maybe warn the users that a LICENSE file could not be located just like the verified publisher.

[isoos]

After randomly sampling 9250 (~28%) of the packages, the following names are present (with random examples):

• LICENSE: 9165 (99%)

• LICENSE.md: 38 (reloadly,stagexl_toolkit,audioplayers_with_rate,data_connection_checker)

• LICENSE.txt: 12 (mubicon,request_extras,fledge,galileo_mustache4dart)

• COPYING: 11 (eric_demo,hex,blockbook,musicus_shared,tba_api_v3,save_image)

• UNLICENSE: 9 (flutter_android,conreality_fonts,openxr,conreality_pulse,model_viewer)

• UNLICENSE.txt: 2 (list_chunker,loading_container)

• COPYING.md: 1 (tinylog)

• missing: 20 (fluttery_cli,stream_binder,energy2d,toastd,pub_test_pkg)

Roughly 1% of the packages are not using LICENSE as the file name, and only 0.2% has no license file (which is a must for new uploads, these packages have no recent version).

[isoos]

the final numbers (out of 32404 packages) are not that different in percentages:

• LICENSE: 32122 (99.1%)

• LICENSE.md: 144 (0.44%) (flutter_boost,qrpc_client,memory_image_converter,flood,flutter_mix,itch_io,at_persistence_spec,libprimesieve,directory_picker,disposables) LICENSE.txt: 38 (0.12%)(mustache4dart2,twitter_1user,tele_bot_api,hestia,galileo_mustache4dart,play_with_words,okhttp,telegram_bot_api,spaceinvaders,lowdb)

• UNLICENSE: 29 (conreality_pulse,supreme_camera,flutter_sqlcipher,flutter_android,near,edropinapi_accounts,hello_example,flutter_lua,flutter_face_detection,accounts)

• COPYING: 27 (musicus_server,libjson2builtvalue,yui,iban_form_field,suisse,flutter_particles,tba_api_v3,flutter_particle,sunmi,bitcoin_bip44)

• UNLICENSE.txt: 5 (list_chunker,image_picker_container,over_screen_widget,polyline_builder,loading_container)

• COPYING.md: 2 (tinylog,watch_recursively)

• missing: 62 (0.19%)(publish_test_console,parse_server,built_union_test,DartDemoCLI,polymerize,zhopa,dart2ts,pub_test_pkg,flutter_bloc_architecture,zengen)

Roughly 0.9% of the packages are not using LICENSE as the file name.