[aot repositories] improve generation of aot metadata

[martinlippert]

At the moment, the mechanism just reads the aot repository metadata file when it is around, but we don't have any support to actually get the metadata file generated. It is automatically generated as a side product of running the build of the project in aot-mode, but we can/should probably provide an easier way to achieve this from within the IDE - and also probably when changing source files of the project, so that those information gets updated quickly.