Wconf flag for configurable warnings [ci: last-only

[lrytz]

(This PR includes the commits of #8338, I'll rebase once that is merged).

Warnings are assigned a category.

Warnings can be filtered by category, by site where they are issued, and by source path (TODO). Deprecations can additionally be filtered by origin (deprecated definition) and since version (TODO).

Filtered can be reported as error, warning, info, summary (like deprecations) or silent.

scalac -Wconf:help
Configure compiler warnings.
Syntax: -Wconf:<filter>&...&<filter>:<action>,<filter>:<action>,...

<filter>
  - any message: any
  - message categories: cat=deprecation, cat=lint, cat=lint-infer-any
  - message content: msg=regex
  - site where the warning is triggered: site=my.package.*
  - source file name: src=src_managed/*
  - origin of deprecation: origin=external.package.*
  - since of deprecation: since<*1.24

<action>
  - error / e
  - warning / w
  - warning-summary / ws
  - warning-verbose / wv (show warning category and site)
  - info / i
  - info-summary / is
  - info-verbose / iv
  - silent / s

The default configuration is:
  -Wconf:cat=deprecation:ws,cat=feature:ws,cat=optimizer:ws

User-defined configurations are added to the left. The leftmost rule matching
a warning message defines the action.

With `-Wconf:any:warning-verbose`, the compiler prints category, site, origin and since
of every warning to help writing filters.

Examples:
  - change every warning into an error: -Wconf any:error
  - ignore certain deprecations: -Wconf:cat=deprecation&origin=some.lib.*&since<*2.2:s

Full list of message categories:
 - deprecation
 - feature, feature-dynamics, feature-existentials, feature-higher-kinds, feature-implicit-conversions, feature-macros, feature-postfix-ops, feature-reflective-calls
 - java-source
 - lint, lint-adapted-args, lint-constant, lint-delayedinit-select, lint-deprecation, lint-doc-detached, lint-eta-sam, lint-eta-zero, lint-implicit-not-found, lint-inaccessible, lint-infer-any, lint-missing-interpolator, lint-nonlocal-return, lint-nullary-override, lint-nullary-unit, lint-option-implicit, lint-package-object-classes, lint-poly-implicit-overload, lint-private-shadow, lint-serial, lint-stars-align, lint-type-parameter-shadow
 - optimizer
 - other, other-debug, other-match-analysis, other-migration, other-pure-statement, other-shadowing
 - scaladoc
 - unchecked
 - unused, unused-imports, unused-locals, unused-params, unused-pat-vars, unused-privates
 - w-flag, w-flag-dead-code, w-flag-extra-implicit, w-flag-numeric-widen, w-flag-octal-literal, w-flag-value-discard

Note: on the command-line you might need to quote configurations containing `*` or `&`
to prevent the shell from expanding it to a list of files in the current directory.

[lrytz]

argh messed up the last only again :-(

[SethTisue]

replaced by #8373