Target pattern parsing cleanup
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* `TargetPattern#renameRepository` is merged into `TargetPattern.Parser#parse`. This is much less error-prone -- repo mapping should not be an afterthought, but something that should always be applied if possible (sometimes it's not, for example for `blaze query`).
  * This also fixes the bug where calling `native.register_toolchains("//:all")` in `@some_repo//:defs.bzl` registers `@//:all` instead of `@some_repo//:all` (see change in RegisteredExecutionPlatformsTest)
* A new class `SignedTargetPattern` is introduced, which can store whether the pattern is positive or negative (with the `sign` method).
* `TargetPatternValue#keys` is greatly simplified thanks to the changes above; the exception throwing is confined to the parsing step, and the construction of `TargetPatternKey` can happen as a separate step, obviating the whole "skykey or exception" dance.
* Following from the above, the //external package now stores registered toolchains and execution platforms as parsed target patterns, instead of simple strings.

Among other things, this would help implement toolchain registration in bzlmod.

PiperOrigin-RevId: 400720457