hadrian *default* build fails without global `xhtml` package in rare configurations

Summary

Hadrian's default build (that is, without explicit target) fails when the boot compiler doesn't have the xhtml package. This is due to the fact that haddock is built at stage 0, even though it's not required for normal installs.

Steps to reproduce

(sudo... use your own judgement?) ghc-pkg unregister xhtml
./boot && ./configure && ./hadrian/build -j --flavour=quickest --build-root=_build

Rather than building ghc, this results in a build failure complaining about missing xhtml when building haddock:

| Configure package 'haddock'
# cabal-configure (for _build/stage0/ghc/setup-config)
# cabal-configure (for _build/stage0/utils/haddock/setup-config)
hadrian: Encountered missing or private dependencies:
xhtml ==3000.2.*

Error when running Shake build system:
  at action, called at src/Rules.hs:40:19 in main:Rules
  at need, called at src/Rules.hs:62:5 in main:Rules
* Depends on: _build/stage0/bin/haddock
  at apply1, called at src/Development/Shake/Internal/Rules/Oracle.hs:159:32 in shake-0.18.5-602f2cdbd3147204d88e77f8741bf241686d3245c138659b0f8f659103b171f2:Development.Shake.Internal.Rules.Oracle
* Depends on: OracleQ (ContextDataKey (Context {stage = Stage0, package = Package {pkgType = Program, pkgName = "haddock", pkgPath = "utils/haddock"}, way = v}))
  at need, called at src/Hadrian/Oracles/Cabal/Rules.hs:53:9 in main:Hadrian.Oracles.Cabal.Rules
* Depends on: _build/stage0/utils/haddock/setup-config
* Raised the exception:
ExitFailure 1

Background

I believe this is a niche case; the binary distributions of GHC include the xhtml package. I got into this situation because I habitually build GHC from source, and without documentation, so it gets installed without haddock and hence without the xhtml library (at least when using make).

Observations

• The make based build system copes with this situation (but it doesn't seem to associate haddock with a stage at all).
• If I install xhtml (cabal v1-install --global xhtml) then the build succeeds.
• There is no xhtml package in _build/stage0/lib/package.conf.d/ in either case, so xhtml must be picked up as a global package.

Environment

• GHC version used: 8.8.3 or 8.10.1 without xhtml package
• Operating System: Linux

Related

• #18094
• #17986 (closed) (though that one is about the stage1 namespace while this issue is only about the stage0 namespace)