Backports for 1.12

[KristofferC]

Backported PRs:

• Update registries during dev #4186
• Add a pkg message if julia_version is being specified #4331
• Clear the registry cache on Pkg module end #4393
• Fix typo in Pkg.jl #4419
• STDLIBS_BY_VERSION: Check sorted & require same minor #4414
• fall back to fetching more refspects in case the restricted set doesnt find the rev #4441
• install / update registries for app add in the same way as for packages #4443
• fix building packages on app add #4491

Need manual backport:

• Fix deleting in compat mode #4402
• update to newer PrecompileTools approach #4421
• update docs to show how workspaces are used for test deps #4428
• normalize paths when writing a project file #4444
• avoid resolving on add when a compatible package exists in the environment #4445
• fix another URL scheme parsing in the repl mode #4450
• do not create a separate app environment for devved apps #4465
• ensure we do not end up in a situation where both tree hash and path is set #4467
• consistently normalize paths to be relative to the manifest #4463
• move some allocations out of hot loop in the graph simplification in the resolver (propagate_constraints!) #4474
• Improve docs for test-specific deps with workspace approach #4490

Contains multiple commits, manual intervention needed:

• Fix completion at the end of a complete line #4373
• Make julia compat checking respect julia_version #4403
• Adapt delayed delete mechanism #4392
• make .path field consistently be relative manifest and convert to project relative upon writing to a project file #4427
• Fix completion on empty command #4418 (cherry picked from commit 0e123a2)