README: update the package list

dirhash, modfile, module, and semver simply forward to the upstream
packages now, so we should no longer advertise them to users.

txtar is very close to that state, but not quite. Add a note.

README.md

@@ -12,18 +12,17 @@ Contributions are welcome, but please open an issue for discussion first.
 
 Included are the following:
 
-- dirhash: calculate hashes over directory trees the same way that the Go tool does.
 - goproxytest: a GOPROXY implementation designed for test use.
 - gotooltest: Use the Go tool inside test scripts (see testscript below)
 - imports: list of known architectures and OSs, and support for reading import statements.
-- modfile: read and write `go.mod` files while preserving formatting and comments.
-- module: module paths and versions.
 - par: do work in parallel.
-- semver: semantic version parsing.
 - testenv: information on the current testing environment.
 - testscript: script-based testing based on txtar files
 - txtar: simple text-based file archives for testing.
 
+Note that most users of `txtar` should use https://pkg.go.dev/golang.org/x/tools/txtar instead.
+Our package will be replaced by it once https://github.com/golang/go/issues/59264 is fixed.
+
 ### testscript
 
 The most popular package here is the [testscript](https://pkg.go.dev/github.com/rogpeppe/go-internal/testscript) package: