Loading package outside of main module when precompiling could have better error message

[fredrikekre]

Brought up on Zulip; the following package code:

$ cat src/TestPackage.jl
using Example

module TestPackage

end # module

gives a rather bizarre error:

$ julia -e 'using TestPackage'
ERROR: LoadError: ArgumentError: Package Base does not have Example in its dependencies:
- You may have a partially installed environment. Try `Pkg.instantiate()`
  to ensure all packages in the environment are installed.
- Or, if you have Base checked out for development and have
  added Example as a dependency but haven't updated your primary
  environment's manifest file, try `Pkg.resolve()`.
- Otherwise you may need to report an issue with Base
Stacktrace:

Perhaps code outside the main module should be ignored altogether, or have a descriptive error? In any case, the error message above should be improved.

[DilumAluthge]

At the very least, printing the line number of the bad using/import statement would help.