When incorporating the gopls.mod file, the invalidation logic for
workspace module information becomes quite complicated. For example:
 + if a modfile changes we should only invalidate if it is in the set of
   active modules
 + the set of active modules can be provided by either the filesystem or
   gopls.mod
 + if gopls.mod changes, we may gain or lose active modules in the
   workspace
 + if gopls.mod is *deleted*, we may need to walk the filesystem to
   actually find all active modules

Doing this with only concrete changes to the snapshot proved
prohibitively complex (at least for me), so a new workspace type is
introduced to manage the module state. This new abstraction is
responsible for tracking the set of active modules, the workspace
modfile, and the set of workspace directories. Its invalidation logic is
factored out of snapshot.clone, so that it can be tested and to
alleviate some of the growing complexity of snapshot.clone.

The workspace type is idempotent, allowing it to be shared across
snapshots without needing to use the cache. There is little benefit to
the cache in this case, since workspace module computation should be
infrequent, and the type itself consumes little memory.

This is made possible because the workspace type itself depends only on
file state, and therefore may be invalidated independently of the
snapshot. The new source.FileState interface is used in testing, and so
that the workspace module may be computed based on both the session file
state as well as the snapshot file state.

As a result of this change, the following improvements in functionality
are possible:
 + in the presence of a gopls.mod file, we avoid walking the filesystem
   to detect modules. This could be helpful for working in large
   monorepos or in GOPATH, when discovering all modules would be
   expensive.
 + The set of active modules should always be consistent with the
   gopls.mod file, likely fixing some bugs (for example, computing
   diagnostics for modfiles that shouldn't be loaded)

For golang/go#41837

Change-Id: I2da888c097748b659ee892ca2d6b3fbe29c1942e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/261237
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Trust: Robert Findley <rfindley@google.com>

gopls.mod files should take precedence over go.mod files when finding a
workspace root. To allow this, implement our own algorithm for expanding
the workspace, rather than using the go command.

For golang/go#41837

Change-Id: I943c08bdbdbdd164f108e44bae95d2c659a6e21e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/263897
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
Trust: Robert Findley <rfindley@google.com>