Be more robust to store corruption

[ezyang]

The new-build code assumes that whatever is in .cabal/store is well-formed. However, in some cases the store can get corrupted; for example, if you upgrade to a new version of cabal-install that uses a different format, or if you manually modify files inside the store. In cases like this, new-build works extremely poorly; it will fail in a supremely uninformative way, and users won't have any recourse except to blow their store away and try again. It would be better if cabal-install was coded defensively against problems in the store.

Examples:

• Self-bootstrap with latest master and index doesn't work #4113 Self-bootstrap with latest master and index doesn't work, it turned out that deleting store fixed the problem
• If some packages in the store are broken, new-build will still attempt o improve to them.