Defer merging of common stanzas

[leana8959]

This is the second part of the exact print parser. We changed the parser so that it doesn't merge the common stanzas during the parsing stage.

Background

Common stanzas are merged into their corresponding section during the parsing stage. This merging process doesn't retain the definition of the imports (i.e. common <name> ...) nor where they are used (import: <name>). To achieve exact printing and to be able to programmatically update the import list, we need to retain this information.

This PR is to remedy exactly that by deferring the merging process.

Implementation

We insert a new field gpdCommonStanza :: Map ImportName (CondTree ConfVar [Dependency] BuildInfo) in the GenericPackageDescription data type to track all the defined common stanzas. The merging is run only when the new accessors are called on a GenericPackageDescription.
We also added a bidirectional PatternSynonym that uses the old record syntax. The impact on existing code should be very low.

Please let me know your thoughts :)

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