caching: don't forget previous invocations

[bjaglin]

Fixes the most annoying caching shortcoming described in scalacenter/scalafix#1164 and discussed in https://gitter.im/scalacenter/scalafix?at=5ea6b3b43d7e50071c34fd67 (1).

This allows cache accumulation after successive runs of scalafix with different rules, which is common as it's currently the only
way to safely run several semantic rules in order.

Note that the cache remains incremental only for files, so if a file is stale for a single rule, all rules will be executed on this file.

Example on a real project

Stamping before

streams
├── args
├── out
└── targets

==> streams/outputs <==
[{"file":"file:///home/bjaglin/git/teads/service-api-domains/framework/impl-commons/src/main/scala/tv/teads/domains/commons/bus/InMemorySynchronousEventBus.scala","lastModified":1593073177421},{"file":"file:///home/bjaglin/git/teads/service-api-domains/fra...

Stamping after

streams
├── args
├── out
└── targets-by-rule
    ├── DisableDefaultValuesForChimney
    ├── ExtendsAbstractHandler
    ├── ExtendsDoobieStatementsLogger
    ├── ImportCustomEitherValuesWithRichErrorMessage
    ├── NoDummyIsAuthorized
    ├── NoEitherRight
    ├── NoSymbolLiteral
    ├── OrganizeImports
    └── RemoveUnused

==> streams/tagets-by-rule/ExtendsAbstractHandler <==
[{"file":"file:///home/bjaglin/git/teads/service-api-domains/framework/impl-commons/src/main/scala/tv/teads/domains/commons/bus/InMemorySynchronousEventBus.scala","lastModified":1593073177421},{"file":"file:///home/bjaglin/git/teads/service-api-domains/fra...

[bjaglin]

This is particularly useful when combined with scalafixOnCompile := true. Without this, a single scalafixAll RandomRuleIAmTrying would invalidate all caches warmed up and used by the scalafix run triggered after each compile.

[bjaglin]

Rebased against #145 - I thought it would make implementation easier, but it does not.

[bjaglin]

@olafurpg I imagine the implementation is getting quite complex to review, but maybe you want to have a look at the tests at least? Thanks!

[olafurpg]

LGTM 👍