Lazily process source maps

Launching devtools can be painfully slow for applications with a significant number of sources, e.g. 20k+. One of the reasons is that we eagerly process source maps to cache metadata. We should be able to make this work lazy.

Start by tracing devtools launch and the eager source-map metadata processing described in the issue, then profile startup with applications containing many sources. Done means source-map work is deferred until needed and launching remains responsive for large source sets.

Written by the indexing model from the issue text.