cmd/internal/testdir: add a -gomodversion flag

Adds a -gomodversion flag to testdir. This sets the go version
in generated go.mod files. This is just runindir tests at the moment.
This is a building block so that tests can be written for exported
type parameterized aliases (like reproducing #68526).

This also adds a test that uses this feature. A type parameterized
alias is used so aliastypeparams and gotypesalias must be enabled.
gotypesalias is enabled by the go module version. The alias is not
exported and will not appear in exportdata. The test shows the
package containing the alias can be imported. This encapsulates
the level of support of type parameterized aliases in 1.23.

Updates #68526
Updates #68778

Change-Id: I8e20df6baa178e1d427d0fff627a16714d9c3b18
Reviewed-on: https://go-review.googlesource.com/c/go/+/604102


LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Austin Clements <austin@google.com>