Add ability to activate test/build target envs

[IanButterworth]

Closes #3460

(Pkg) pkg> activate /Users/ian/Documents/GitHub/ObjectDetector.jl
  Activating project at `~/Documents/GitHub/ObjectDetector.jl`

(ObjectDetector) pkg> activate --test
    Creating temporary project based on the target `test` in `~/Documents/GitHub/ObjectDetector.jl/Project.toml`
  Activating project at `/var/folders/1z/jf841bdj73bdj3vk7kc7f_3w0000gn/T/jl_wHY4q9`

(jl_wHY4q9) pkg> st
Status `/private/var/folders/1z/jf841bdj73bdj3vk7kc7f_3w0000gn/T/jl_wHY4q9/Project.toml`
  [79e6a3ab] Adapt
  [5c00bae2] AllocArrays
  [6e4b80f9] BenchmarkTools
  [052768ef] CUDA
  [e2912957] Darknet
  [5789e2e9] FileIO
  [587475ba] Flux
  [a09fc81d] ImageCore
  [4381153b] ImageDraw
  [6a3955dd] ImageFiltering
  [82e4d734] ImageIO
  [02fcd773] ImageTransformations
  [aea7be01] PrecompileTools
  [08abe8d2] PrettyTables
  [324d217c] ReferenceTests
  [a759f4b9] TimerOutputs
  [c4a57d5a] UnsafeArrays
  [02a925ec] cuDNN
  [4af54fe1] LazyArtifacts
  [8dfed614] Test

TODO:

• Fix including the parent package in the temp env

[davidanthoff]

This would be amazing to have! Lots of use for it in the VS Code extension. Probably too late for 1.12?

[IanButterworth]

@KristofferC mentioned in #3460 that the new workspaces feature was a better way to handle this in 1.12+

We could add this functionality, or we could not and focus on prodding people towards using workspaces in 1.12+.

It came up in the #testing slack channel that this section of the docs should be updated to highlight workspaces being the best way to do test envs. https://pkgdocs.julialang.org/v1/creating-packages/#Alternative-approach:-test/Project.toml-file-test-specific-dependencies

[IanButterworth]

Also so I don't forget if we go with this PR.. there's a bug. It's currently not including the parent package in the test env.

[davidanthoff]

It would still be great to have just one function that handles all the complexities of deciding how to set up the test env. I.e. if the workspace is there, is that, if the old style separate Project.toml is there, us that, or the target thing. We will essentially have three different ways to specify test env in 1.12 that all need to be supported, and it would be nice if Pkg.jl handled all cases.

[Moelf]

with workspaces, this should be expanded to be able to activate any workspace or...? --docs would be useful, and --benchmarks