Support trimming artifacts for packages that were never loaded

[topolarity]

In theory (since --trim restricts top-level code from ever executing after (pre-)compilation), JuliaC should be able to:

1. Query the list of loaded packages
2. Generate a LoadedManifest.toml for PackageCompiler
3. Enable some feature in PackageCompiler to trim the set of included artifacts / code based on the LoadedManifest.toml
   (which is a strict subset of your full Manifest.toml, if there were packages that were never using'd)

This should mostly be a matter of generating the smaller manifest and then having PackageCompiler use that manifest instead of the normal one for discovering code / artifacts to download / include.

[vishal2005025]

i want to work on this issue.