Improve startup performance by lazy-loading extensions

[amykyta3]

Currently all extensions are discovered and imported unconditionally upon each invocation of the tool. This is not great.

Instead, rework the command-line processor so that the specific exporter plugin is only loaded if invoked.

There are some other situations where all extensions may need to be loaded, but thats limited to stuff like running the top-level --help command, or dumping plugin versions (may be able to avoid)