[LTO] Turn ImportListsTy into a proper class (NFC) (#106427)

This patch turns ImportListsTy into a class that wraps
DenseMap<StringRef, ImportMapTy>.

Here is the background.  I'm planning to reduce the memory footprint
of ThinLTO indexing.  Specifically, ImportMapTy, the list of imports
for a given destination module, will be a hash set of integer IDs
indexing into a deduplication table of pairs (SourceModule, GUID),
which is a lot like string interning.  I'm planning to put this
deduplication table as part of ImportListsTy and have each instance of
ImportMapTy hold a reference to the deduplication table.

Another reason to wrap the DenseMap is that I need to intercept
operator[]() so that I can construct an instance of ImportMapTy with a
reference to the deduplication table.  Note that the default
implementation of operator[]() would default-construct ImportMapTy,
which I am going to disable.