Clean up plugin build process

[helin24]

While reading about plugin structure as part of decisions around dart plugin, we decided to reorganize the flutter plugin as well (mostly from @jwren). The goal is to make plugin building easier to understand and easier to debug by following conventions of IntelliJ plugin setup and gradle and removing complex paths that were added for historical reasons but are no longer needed.

• [x] remove build files in flutter-idea and flutter-studio subdirectories - instead of using a module structure for intellij vs Android studio plugins, we should be able to specify code to include/exclude in plugin.xml files https://github.com/flutter/flutter-intellij/pull/8278
• [x] flatten directories to remove flutter-idea and flutter-studio (https://github.com/flutter/flutter-intellij/pull/8343, https://github.com/flutter/flutter-intellij/pull/8384)
• [x] Remove required Android studio plugin dependency https://github.com/flutter/flutter-intellij/pull/7949
• [x] Consider how to support multiple platform versions - this could mean generating a single plugin that spans versions, using branches, or keeping the status quo (product-matrix.json). The complex build process for multiple versions was more necessary when platform APIs were changing quickly, but may not be needed now. (https://github.com/flutter/flutter-intellij/pull/8398)
• [x] Reduce dependency on plugin.dart and related files - we are doing a lot of customized file reading and writing and other tasks that could likely be replaced with gradle and IntelliJ platform configuration. (https://github.com/flutter/flutter-intellij/pull/8360, https://github.com/flutter/flutter-intellij/pull/8363, https://github.com/flutter/flutter-intellij/pull/8387, https://github.com/flutter/flutter-intellij/pull/8400, https://github.com/flutter/flutter-intellij/pull/8407, https://github.com/flutter/flutter-intellij/pull/8411)
• [x] Use more conventional directory structure (https://github.com/flutter/flutter-intellij/pull/8397)

Related cleanup - https://github.com/flutter/flutter-intellij/pull/8326, https://github.com/flutter/flutter-intellij/pull/8327, https://github.com/flutter/flutter-intellij/pull/8416, https://github.com/flutter/flutter-intellij/pull/8424