Package manager nullability fixes

[50Wliu]

Covers:

• src/packageManager/
• src/razor/
• src/tools/

Notable changes:

• Added missing architectures to macOS x64 OmniSharp download (I hope this wasn't intentional to allow M1 Macs to download both ARM64 + x64 OmniSharps? architectures is marked as non-null in IPackage).
• Remove all guards that checked for non-null packages, as there is a root check for that when initially reading runtimeDependencies from package.json.
  • This is slightly risky as it's possible I missed a root caller that doesn't check that runtimeDependencies from an any-typed package.json is non-null - but I mean, with the previous behavior we would just silently refuse to download OmniSharp. At least now there will be an obvious error thrown somewhere.
• Similarly, removed all version checks as there's a root semver.valid(version) check.
• downloadAndInstallPackages is only ever called with a list of validated OmniSharp packages (function should be renamed to reflect that...), so removed some unnecessary checks around that.

While we're here, I'd like to understand the current state of how runtimeDependencies are defined/parsed, so that I have the context to create a followup PR cleaning up some of the associated logic.

• Is installTestPath used for anything? There's 0 references from AbsolutePathPackage and I don't see any meaningful reads from IPackage.
• Why do the packages in package.json specify a URL + installPath, if they're just overriden in OmnisharpPackageCreator (albeit, to the same thing) from constants in server.ts?
  • Should we stop overriding in OmnisharpPackageCreator, or stop including the URL + installPath in package.json?
• fallbackUrl is never defined in any of the packages in package.json. Is this something that should be kept around?

[JoeRobich]

Thanks @50Wliu! Sorry it took me so long to review.

[JoeRobich]

Thanks for removing this.