--share option for hadrian doesn't work with hs-boot files

[Matthew Pickering]

I have been trying the new `--share` option implemented in hadrian but I
haven't actually managed to
complete any builds yet with it enabled after the initial one.

The current error is

```
<no location info>: error:
    ‘Var.AnonArgFlag’ is exported by the hs-boot file, but not exported by
the module
Error when running Shake build system:
  at action, called at src/Rules.hs:35:19 in main:Rules
  at need, called at src/Rules.hs:52:5 in main:Rules
* Depends on: _build/stage0/bin/ghc
  at need, called at src/Utilities.hs:71:18 in main:Utilities
* Depends on: _build/stage0/compiler/build/libHSghc-8.9.a
  at need, called at src/Rules/Library.hs:118:5 in main:Rules.Library
* Depends on: _build/stage0/compiler/build/Var.o
* Raised the exception:
user error (Development.Shake.cmd, system command failed
```

I get this after building Simon's `FunTy` patch which does add this flag
and the definition to `Var.hs-boot` and then switching back to master with
the cache enabled.

Could you please write down some advice Andrey about how to solve issues
like this? It seems very fragile making sure that every case is covered.

Cheers,

Matt
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