x/tools/gopls: watch replaced directories

[findleyr]

At some point in the past, I'm pretty sure gopls registered file watchers for directories that were explicitly replaced via go.mod replace directives. However, this logic was reverted at some point, most likely while adding support for go.work files.

The rationale for watching replaced directories still makes sense to me, and so we should add it back, with tests.

[gopherbot]

Change https://go.dev/cl/562680 mentions this issue: gopls/internal/cache: treat local replaces as workspace modules