Privatize almost all elements in package_builder.

[srawlins]

Everything in this library is basically library-private: make it so:

• Privatize config, packageMetaProvider, packageConfigProvider, sdk, embedderSdk, pathContext, processLibrary (and rename to _resolveLibrary), getFiles (and rename to _getFilesToDocument), getEmbedderSdkFiles (and refactor as a getter named _embedderSdkFiles), hasEmbedderSdkFiles. Phew!

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