Prior to this PR, Julia's precompiled `*.ji` files saved just two
categories of code: unspecialized method definitions and
type-specialized code for the methods defined by the package.  Any
novel specializations needed from other code (Base, other packages)
were not saved, and therefore effectively thrown away.

This PR caches all the code---internal or external---called during
package definition that hadn't been previously inferred. This makes
precompilation more intuitive (now it saves all relevant inference
results), and substantially reduces latency for inference-bound
packages.

Closes #42016
Fixes #35972

Issue #35972 arose because codegen got started without re-inferring
some discarded CodeInstances. This forced the compiler to insert a
`jl_invoke`. This PR fixes the issue because needed CodeInstances are
no longer discarded by precompilation.

Also adds a more stringent invalidation test

This should prevent us from serializing too much code.