cmd/cover: carve out deprecated command into its own module

Since cmd/cover imports x/tools/cover (which is not itself
deprecated), its go.mod file needs to require a version of x/tools
that includes cover but not cmd/cover.
Arbitrarily choose x/tools v0.6.0 as the likely next x/tools release
tag and slot it in using a 'replace' directive.

In a followup commit, I will remove the replace directive and update
the x/tools dependency to the pseudo-version of the exact commit of
this CL.

For golang/go#56783.

Change-Id: Ia70206cb8c4e6128b90a048aad4e2607db19740b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/461655
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

cmd/cover/go.mod

@@ -0,0 +1,10 @@
+module golang.org/x/tools/cmd/cover
+
+go 1.18
+
+replace golang.org/x/tools v0.6.0 => ../..
+
+require (
+	golang.org/x/sys v0.4.0
+	golang.org/x/tools v0.6.0
+)

cmd/cover/go.sum

@@ -0,0 +1,2 @@
+golang.org/x/sys v0.4.0 h1:Zr2JFtRQNX3BCZ8YtxRE9hNJYC8J6I1MVbMg6owUp18=
+golang.org/x/sys v0.4.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=