Add --pkgimages=existing. (#52573)
Equivalent of https://github.com/JuliaLang/julia/pull/50586; implements
https://github.com/JuliaLang/julia/issues/51474.
With `--pkgimages=existing`, it's possible to disable the (often slow)
generation of package images, without losing the ability to use existing
ones. That's important now that we're moving more and more packages
outside of the system image, e.g., running with `--pkgimages=no`
otherwise takes close to 30s here before the Pkg REPL is usable.
The main motivation for this is PkgEval, where generating package images
is not very useful, yet disabling generation of them makes each job
(which requires Pkg to drive the test process) take a significantly
longer time.
For example, `--pkgimages=yes` vs `no`:
```julia
❯ JULIA_DEBUG=loading ./julia --project=Example.jl --pkgimages=yes
# no precompilation of REPL.jl
┌ Debug: Loading object cache file /Users/tim/Julia/src/julia/build/dev/usr/share/julia/compiled/v1.11/REPL/u0gqU_XmENM.dylib for REPL [3fa0cd96-eef1-5676-8a61-b3b8758bbffb]
└ @ Base loading.jl:1116
julia> using Example
# short time precompiling + pkgimg generation for Example.jl
┌ Debug: Loading object cache file /Users/tim/.julia/compiled/v1.11/Example/lLvWP_tJaso.dylib for Example [7876af07-990d-54b4-ab0e-23690620f79a]
└ @ Base loading.jl:1116
```
```julia
❯ JULIA_DEBUG=loading ./julia --project=Example.jl --pkgimages=no
┌ Debug: Rejecting cache file /Users/tim/Julia/src/julia/build/dev/usr/share/julia/compiled/v1.11/REPL/u0gqU_XmENM.ji for REPL [3fa0cd96-eef1-5676-8a61-b3b8758bbffb] since the flags are mismatched
│ current session: use_pkgimages = false, debug_level = 1, check_bounds = 0, inline = true, opt_level = 2
│ cache file: use_pkgimages = true, debug_level = 1, check_bounds = 0, inline = true, opt_level = 2
└ @ Base loading.jl:3289
# long time precompiling REPL.jl
┌ Debug: Loading cache file /Users/tim/.julia/compiled/v1.11/REPL/u0gqU_CWvWI.ji for REPL [3fa0cd96-eef1-5676-8a61-b3b8758bbffb]
└ @ Base loading.jl:1119
julia> using Example
# short time precompiling Example
┌ Debug: Loading cache file /Users/tim/.julia/compiled/v1.11/Example/lLvWP_CWvWI.ji for Example [7876af07-990d-54b4-ab0e-23690620f79a]
└ @ Base loading.jl:1119
```
With the new `--pkgimages=existing`:
```julia
❯ JULIA_DEBUG=loading ./julia --project=Example.jl --pkgimages=existing
# no precompilation of REPL.jl
┌ Debug: Loading object cache file /Users/tim/Julia/src/julia/build/dev/usr/share/julia/compiled/v1.11/REPL/u0gqU_XmENM.dylib for REPL [3fa0cd96-eef1-5676-8a61-b3b8758bbffb]
└ @ Base loading.jl:1116
julia> using Example
# short time precompiling Example
┌ Debug: Loading cache file /Users/tim/.julia/compiled/v1.11/Example/lLvWP_CWvWI.ji for Example [7876af07-990d-54b4-ab0e-23690620f79a]
└ @ Base loading.jl:1119
```
