Expose some useful type/func to pkg to be called by external Go modules

[magodo]

This PR exposes following types/functions to pkg to be able to be depended by external Go modules, but with no compatibility guarantees at this moment. This is mainly to be depended by another internal project for now:

• pkg/conifg: The configuration type Config
• pkg/log: The type and function for setting up logger
• pkg/meta: The main interface Meta and its builder function

Besides, this PR adds following fixes/enhancements:

• Fix a bug introduced due to using os.Chdir in acctest
• Removes the use of f.Sync (because of above fix), which is not a right thing to do
• Improve the ParallelImport to allow a list longer than parallelism
• New NonInteractiveModeConfig and InteractiveModeConfig to better reason about the configurations given the Config is exported, we put (non)interactive related config to these new types on top of Config, which should not be exported to external users
• The methods of Meta now accepts context.Context