tweak how filtering is done for what packages should be precompiled

[KristofferC]

Due to a bug in Pkg (JuliaLang/Pkg.jl#3863) Pkg sometimes generates non-pruned manifests. When package precompilation encountered this it still somehow tried to precompile these orphan packages. To me, it seems that the filtering that the precompilation system does to only precompile requested packages comes a bit late when even extensions have been entered into the "depsmap".

This moves the filtering up earlier so that it happens at the creation of depsmap and from there on we see a "pruned view" of the environment and carry on like normally.

[IanButterworth]

Sounds good. Given this is working around a bug it's probably worth adding a test with a manifest with a bad entry.

[KristofferC]

Given this is working around a bug i

With workspaces I would argue this is a bugfix since you can have "orphan" entries in the manifest when looked at from the active projects p.o.v that are not orphan when looked at from the whole workspace p.o.v.

However, I am not clear on the exact conditions required to provoke this. My guess is that the package has a dependency that has a weak dependency that is the orphan package... Or something like that.

[KristofferC]

The assumption here is that the packages given to precompilepkgs are packages in the project file since those are the only ones you can load from Main but this is not completely true, it can also end up being an extension when the require callback fires and the extension has not been precompiled.