[NFC, Incremental] Refactoring to facilitate replacing the inputDependencySourceMap with the right thing

[davidungar]

Redo of reverted #664 ; reverted because I forgot to add a new file to CMakeLists.txt.

This PR paves the way for bug fixes by adding an intentions layer to the usage of a bidirectional map to go from input files (.swift) to and from dependency files (.swiftdeps).

1. Wrap the BidirectionalMap into its own type, and put that into InputDependencySourceMap.swift
2. Make the interface very explicit as to intentions:
   • Add an enum to explain why entries are being added (AdditionPurpose) and pass that through calls that add entries
   • addMapEntry in the ModuleDependencyGraph becomes addEntry on the new type
   • The new type does lookups via sourceIfKnown(for:) and inputIfKnown(for:) to contrast the possibility of failure with sourceRequired(for:), which is the new name for getSource(for:) in the graph method.
3. Fixes a misnomer in a local variable in collectInputsInvalidatedBy(input:)
4. The reduce in populateInputDependencySourceMap has been rewritten for clarity.

In preparation for: rdar://77998890

[davidungar]

@swift-ci please test

[davidungar]

Thanks, @akyrtzi !