Make project import interactive

Right now, importing Java projects is happening at startup. I am aware that there is the "interactive" mode, but all that allows me to do is to veto the import of the projects. (If I select "later", how do I later import the projects? There seems to be no "Import Projects" command 🤷 )

Let's consider making the import more interactive by

• allow the user to pick which directories will be imported
• allow the user to select which import will import a directory is two or more want to import a given directory
• allow importers to ask the user to fix problems while importing (think "no compatible Java version" could be fixed by selecting a JDK directory).

In order to implement something like this, we'd have to move control of the import process to the vscode-java side instead of handling import in the language server. That would also alllow us to give more precise feedback if something goes wrong with the import. IMO.

Start by tracing the startup import flow, the existing interactive mode, and the boundary between the VS Code extension and language server. Define how users choose directories and importers, resolve importer problems, and later trigger imports; done means these interactions provide precise feedback without relying on startup-only importing.