internal/modindex: memoize the index parsed by Read

Read parses the entire on-disk index, which may be hundreds of
megabytes, on every call. Clients that format many files call it once
per file: imports.Process constructs a fresh ProcessEnv for each call,
so nothing can be carried over between files, and getFixesWithSource
consults the index for every file that has an unresolved reference.

CL 811860 moved that call past the early returns, so a file whose
imports are already complete no longer touches the index. A file that
does need a candidate from the module cache still causes the whole
payload to be parsed again.

Memoize the parsed index, keyed by the name of the payload file it was
read from. Payload files are immutable: write gives each one a fresh
random name and only then updates the link file to refer to it. The
name is therefore a sound cache key, an index written by another
process is picked up as soon as the link file changes, and only the
most recent index for each module cache directory is retained.

An Index may now be shared by several callers and must not be
modified. Neither caller in this repository does: build copies the
entries of the previous index into a new slice, and gopls holds the
pointer under a mutex and only ever replaces it.

goimports over 100 files that each need one import added, with a 54MB
index in the module cache:

      before  12.6s
      after    1.3s

and over 463 files of a real project with their imports stripped:

      before  52.9s
      after   13.7s

Updates golang/go#80774

Change-Id: I4dfa0f2e354af4db00c0177e1c5c84f6ab04b5c1
Reviewed-on: https://go-review.googlesource.com/c/tools/+/817520
Reviewed-by: Peter Weinberger <pjw@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Hongxiang Jiang <hxjiang@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>