Make incremental program loading

[jirfag]

Currently, it's the slowest part of golangci-lint in "slow" (without --fast) mode.
Need to cache loaded packages and reload only changed packages.

[tpounds]

@jirfag Is this still valid after all the recent work in #337?

[jirfag]

yes, it's valid, I'm currently doing that

[tpounds]

Sounds good! I'll remove the stale label.

[jirfag]

done in #768