The code in TestEnv.jl is made from hacking up the internal code from Pkg.
This means it often breaks, due to changes in Pkg that are not mirrored in TestEnv yet.
Though for the last few releases the Pkg internals seem to be back in a period of stability.

I think at this point, TestEnv.jl has proven the utility of being able to activate the test enviroment.
And that it would be reasonable to add ] activate --test or similar.
If we did that and stdlib-ized the the TestEnv functionality then it wouldn't need
Could keep TestEnv.jl around for compat reasons, but it could just delegate to Pkg if version is beyond 1.10

An alternative would be to expose just enough functionality to do this.
This probably means a public API of:

• extract UUID from [extras]
• easily get lists of packages from target.test
• and easily subset [compat]
• Merge Projects/Manifests