dap: await source map population before starting debugger

[mrcjkb]

          :thinking: 

The reason I use state is because the callback passed to vim.system is asynchronous in Neovim 0.10, so the source tables may not be ready by the time the user is presented with a choice of debuggables (but it should be by the time a choice is made). This makes the UI feel a bit more snappy (although I'll probably want to add some await calls before actually starting the debugger, to be on the safe side).

The way it's implemented right now - with only one state table - it's possible for multiple projects to interfere with each other. We should probably at least have a table with the workspace root as the keys to prevent this.

Originally posted by @mrcjkb in #64 (comment)

[mrcjkb]

Going with YAGNI on this.