Fix #412: Protect the state of packages and loaders from exceptions.

[sjrd]

At the loader level:
Only remove package data entries after successful loading.

At the package symbol level:
While loading new symbols from the loader, all new declarations are put in a pendingDeclarations safe zone. They are committed to myDeclarations only after the whole operation successfully complets.

This way, if a failure happens while reading one root, it does not corrupt the other roots in the same package.

---

Locally tested with

    libraryDependencies += "dev.zio" %% "zio-test" % "2.0.16",

and the following TypeSuite test:

  testWithContext("issue-412") {
    val zioPackage = ctx.findPackage("zio")

    // Cause the initial failure
    intercept[TastyFormatException](zioPackage.declarations)

    val decl1 = zioPackage.findDecl(typeName("ZInputStream")) // before problematic file
    assert(decl1.isClass)

    val decl2 = zioPackage.findDecl(typeName("ZOutputStream")) // after problematic file
    assert(decl2.isClass)

    val decl3 = zioPackage.findDecl(termName("package")).asTerm // original issue
    assert(decl3.isModuleVal)

    // Trying to read the problematic file still reports the same exception
    intercept[TastyFormatException](zioPackage.findDecl(termName("ZLayerCompanionVersionSpecific")))

    // Trying to load everything again still reports the same exception
    intercept[TastyFormatException](zioPackage.declarations)
  }

[sjrd]

Hum this is still broken. I'll fix it tomorrow.

[sjrd]

OK I believe this really good, now. I enhanced the test in the PR description.

[adpi2]

Actually there is a little regression in this. Before it would add all the symbols in the file up to the exception, but now it does not add any. Is this the desired behavior?

object Foo

class Foo

I cannot find the object Foo anymore even if the exception is in the TASTy part of class Foo.

[sjrd]

Yes, that's expected. A companion and its class may have symbolic links between each other. If we throw away one but keep the other, it may contain and expose references to Symbols that were never properly completed.