refactor: Update in-memory Lockfile type to v6 inline specifiers format

[gluxon]

In #5091, a new use-inline-specifiers-lockfile-format config was added. This was folded into the v6 lockfile format #5810, which pnpm v8 now uses by default.

For the migration process, pnpm uses the v5 lockfile format in-memory and converts to/from the v6 format when (de)serializing the lockfile.

Here's where pnpm current converts the lockfile back to the old v5 format:

pnpm/lockfile/lockfile-file/src/write.ts

Line 77 in 158d8cf

const lockfileToStringify = isLockfileV6

Removing this conversion should clean up a bit of tech debt. It'll be a bit involved since it requires updating any code that depends on the Lockfile schema.

Note that we'll still need some conversion until the next major version. pnpm v8 should understand the older lockfile format so it's able to update older pnpm-lock.yaml files.

[gluxon]

@zkochan Just confirming we want to do this. Wanted to make sure this was filed so I could clean up tech debt I introduced.