Flutter pub get does not take into account submodules/ projects

## Steps to Reproduce
1. Create a project
2. Create a submodule/package that is its own dart library.
3. Add something to the submodule's pubspec.
4. Open a different file from the submodule and click `Get Dependencies` when the option shows up.
5. Notice that `flutter pub get` is run from the top-level project, and not from the one the dart file that was open was run from.
6. Suffer and go into the terminal to run it manually

## Version info

[✓] Flutter (Channel stable, v1.17.5, on Mac OS X 10.15.5 19F101, locale en-US)
• Flutter version 1.17.5 at /Users/thinkdigital/development/flutter_stable
• Framework revision 8af6b2f038 (3 weeks ago), 2020-06-30 12:53:55 -0700
• Engine revision ee76268252
• Dart version 2.8.4

[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.0-rc4)
• Android SDK at /Users/thinkdigital/Library/Android/sdk
• Platform android-30, build-tools 30.0.0-rc4
• ANDROID_HOME = /Users/thinkdigital/Library/Android/sdk
• ANDROID_SDK_ROOT = /Users/thinkdigital/Library/Android/sdk
• Java binary at: /Users/thinkdigital/Library/Application Support/JetBrains/Toolbox/apps/AndroidStudio/ch-0/193.6626763/Android
Studio.app/Contents/jre/jdk/Contents/Home/bin/java
• Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6222593)
• All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 11.5)
• Xcode at /Applications/Xcode.app/Contents/Developer
• Xcode 11.5, Build version 11E608c
• CocoaPods version 1.9.3

[✓] Android Studio (version 4.0)
• Android Studio at /Users/thinkdigital/Library/Application Support/JetBrains/Toolbox/apps/AndroidStudio/ch-0/193.6626763/Android Studio.app/Contents
• Flutter plugin version 48.0.2-dev.3
• Dart plugin version 193.7361
• Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6222593)

Start by reproducing the listed steps in the Flutter/IntelliJ plugin and inspect the `Get Dependencies` action entry point. Done means `flutter pub get` runs from the submodule or package containing the open Dart file; verify the behavior with a project containing a Dart submodule.

Written by the indexing model from the issue text.