cmd/go: make TestScript/mod_get_patchmod self-contained

I find it pretty difficult to reason about test-dependency modules
when they aren't in the same file as the rest of the test.

Now that 'go get' supports replacements (CL 258220 and CL 266018),
we can localize tests that need 'go get' but don't specifically depend
on module proxy semantics.

For #36460
For #37438

Change-Id: Ib37a6c170f251435399dfc23e60d96681a81eadc
Reviewed-on: https://go-review.googlesource.com/c/go/+/266369
Trust: Bryan C. Mills <[email protected]>
Run-TryBot: Bryan C. Mills <[email protected]>
Reviewed-by: Jay Conrod <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: Bryan C. Mills

src/cmd/go/testdata/mod/example.net_pkgremoved_v0.1.0.txt

@@ -17,7 +17,7 @@ stdout '^example.net/pkgremoved'
 
 ! go get -d example.net/pkgremoved@patch example.net/[email protected]
 
-stderr '^go get example.net/pkgremoved@patch: module example.net/pkgremoved@latest found \(v0.2.1\), but does not contain package example.net/pkgremoved$'
+stderr '^go get example.net/pkgremoved@patch: module example.net/pkgremoved@latest found \(v0.2.1, replaced by ./pr2\), but does not contain package example.net/pkgremoved$'
 
 
 -- go.mod --
@@ -27,6 +27,10 @@ go 1.16
 
 replace (
 	example.net/other v0.1.0 => ./other
+  example.net/pkgremoved v0.1.0 => ./pr1
+  example.net/pkgremoved v0.1.1 => ./pr1
+  example.net/pkgremoved v0.2.0 => ./pr2
+  example.net/pkgremoved v0.2.1 => ./pr2
 )
 -- other/go.mod --
 module example.net/other
@@ -36,3 +40,13 @@ go 1.16
 require example.net/pkgremoved v0.2.0
 -- other/other.go --
 package other
+-- pr1/go.mod --
+module example.net/pkgremoved
+
+go 1.16
+-- pr1/pkgremoved.go --
+package pkgremoved
+-- pr2/go.mod --
+module example.net/pkgremoved
+-- pr2/README.txt --
+Package pkgremoved was removed in v0.2.0.