build: almost complete module support (#33)

This is a necessarily large PR to offer almost complete Go modules
support for GopherJS.

* We now use github.com/rogpeppe/go-internal to be able to write testscript
  tests
* A suite of testscript tests have been added to cover the major GopherJS
  commands operating in both GOPATH mode and module mode.  Includes a test to
  ensure that the bug raised in #27 is now properly fixed.
* The GopherJS tool and build packages have been heavily refactored in places to
  support Go modules. GopherJS support for Go modules follows the same patterns
  as the go tool.
* We fix the build cache to be a function of the augmented packages as opposed
  (incorrectly) to the unaugmented package files. This has a slight cost because
  of the way the augmentation code is currently written (this could be refactored
  in a later PR) because every file is unconditionally parsed. Main and test
  packages are not cached (again, we could change this in a later PR). Because it
  would just work.
* We fix the tests/run.go wrapper around the fixedbugs tests in the Go
  distribution to not perform a chdir into GOROOT; this does not work with
  modules.
* We tidy up .circleci/config.yml to split commands into their separate run
  sections where it makes sense to.
* We add genmodstubbs.go to automatically populate the stubs we need in
  testdata/mod for the loading of github.com/gopherjs/gopherjs/{js,nosync}.
* For a more current commentary on the Go module support in GopherJS see
  https://github.com/myitcv/gopherjs/wiki/Changes-in-module-aware-GopherJS

Fixes #24
Fixes #27

.circleci/config.yml

@@ -70,47 +70,17 @@ jobs:
 
           # this gives us the repo-local binaries we require
           npm install
-
-      - run: |
-          go install github.com/gopherjs/gopherjs
-
-      - run:
-          # because this is a long-running test suite
-          no_output_timeout: 30m
-          command: |
-
-              echo ">> Regenerate compiler/prelude"
-              go generate github.com/gopherjs/gopherjs/compiler/prelude
-
-              echo ">> Ensure we have a tidy go.{mod,sum}"
-              go mod tidy
-              go list all > /dev/null # https://github.com/golang/go/issues/27868#issuecomment-431413621
-
-              git diff
-
-              echo ">> Ensure there are no variations from the git commit"
-              diff -u <(echo -n) <(git status --porcelain)
-
-              echo ">> Ensure all go code is well formatted."
-              diff -u <(echo -n) <(gofmt -d .)
-
-              echo ">> Vetting github.com/gopherjs/gopherjs"
-              go vet .
-
-              echo ">> vet all packages except tests"
-              go vet $(go list ./... | grep -v github.com/gopherjs/gopherjs/tests)
-
-              echo ">> All those packages should have // +build js."
-              diff -u <(echo "github.com/gopherjs/gopherjs/compiler/natives") <(go list ./compiler/natives/...)
-
-              echo ">> Should build successfully (can't run tests, since only client is supported)."
-              gopherjs install -v net/http
-
-              echo ">> Run the core gopherjs tests; exclusions take from .std_test_pkg_exclusions."
-              gopherjs test --minify -v --short github.com/gopherjs/gopherjs/tests/... $(go list std | grep -v -x -f .std_test_pkg_exclusions)
-
-              echo ">> Race tests."
-              go test -v -race ./...
-
-              echo ">> Non-minified gopherjs tests should also work."
-              gopherjs test -v fmt
+      - run: go install github.com/gopherjs/gopherjs
+      - run: go generate github.com/gopherjs/gopherjs/compiler/prelude
+      - run: go mod tidy && go list all > /dev/null # https://github.com/golang/go/issues/27868#issuecomment-431413621
+      - run: go run genmodstubbs.go
+      - run: git diff
+      - run: diff -u <(echo -n) <(git status --porcelain)
+      - run: diff -u <(echo -n) <(gofmt -d .)
+      - run: go vet .
+      - run: go vet $(go list ./... | grep -v github.com/gopherjs/gopherjs/tests)
+      - run: diff -u <(echo "github.com/gopherjs/gopherjs/compiler/natives") <(go list ./compiler/natives/...)
+      - run: gopherjs install -v net/http
+      - run: gopherjs test --minify -v --short github.com/gopherjs/gopherjs/tests/... $(go list std | grep -v -x -f .std_test_pkg_exclusions)
+      - run: go test -v ./...
+      - run: gopherjs test -v fmt