cmd/go/internal/modindex: disable indexing for modules outside GOROOT and the module cache
Since CL 410821 we were indexing these modules with a cache key based
on the mtimes of the files within the module. However, that seems to
be causing test failures (#53269 and maybe #53371).
In addition, indexing these modules caused a potentially-expensive
operation (re-indexing a whole module) whenever any individual file
within the module is changed, even if it isn't relevant to the
package(s) being loaded from that module. In some cases, that could
cause a significant performance regression for 'go' commands invoked
on a small subset of the packages in the module (such as running 'go
test' on a single changed package — a common case during development).
Instead, we now index only those modules found within the module cache
and within GOROOT.
In addition, we now check mtimes when indexing GOROOT modules if the
Go version begins with the string "devel ", which indicates a
non-released Go version that may include local file edits within GOROOT.