This PR suggests improving the IntelliJ plugin "jar" detection logic.

Previously:
The IntelliJ Flutter plugin was contained flutter-intellij.jar.
Currently:
It is named flutter-intellij-X.Y.Z.jar and does not contain META-INF/plugin.xml.
META-INF/plugin.xml is included in flutter-idea-X.Y.Z.jar.

So this PR changes the rules for searching the plugin's jar file.
Concretely, it looks for the jar file containing META-INF/plugin.xml in the plugin's package directory and reads the package version from its META-INF/plugin.xml.
Co-authored-by: Jonah Williams <jonahwilliams@google.com>