This optionally allows the parser to massage the file structure into an
alternate output format.

These files are currently not utilized by the backend but they might be
in the near future.

…rposes

Shares a common Parser (or Parsers, one per runtime) within a Session and
avoids unnecessary AST walks for filePaths the parser has already seen when
parsing other entrypoints.

Share PackageFilesResolver so that its file caches can be shared within the
same Parser (which is now also shared within the Session), and cache
its result per filePath to avoid duplicate work.

Helps use cases where there are multiple entrypoints that share
common libraries.

Does not play well during a nested lookup when we're looking up a path that
we've already resolved to a candidate with a .ts extension earlier in the
process. Not a super useful feature anyway.