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 v0.4.0 as the likely next x/tools release
tag and slot it in using a 'replace' directive.

For golang/go#56783.

Change-Id: I26a5829d0421724ab14437955a8cec4acbfd62b0
Reviewed-on: https://go-review.googlesource.com/c/tools/+/451595
Run-TryBot: Bryan Mills <[email protected]>
gopls-CI: kokoro <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>

Author: Bryan C. Mills

cmd/cover/go.mod

@@ -0,0 +1,10 @@
+module golang.org/x/tools/cmd/cover
+
+go 1.18
+
+replace golang.org/x/tools v0.4.0 => ../..
+
+require (
+	golang.org/x/sys v0.2.0
+	golang.org/x/tools v0.4.0
+)

cmd/cover/go.sum

@@ -0,0 +1,2 @@
+golang.org/x/sys v0.2.0 h1:ljd4t30dBnAvMZaQCevtY0xLLD0A+bRZXbgLMLU1F/A=
+golang.org/x/sys v0.2.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=