go/types: in TestStdlib, import from source instead of export data

TestStdlib was failing after running
	rm -r $(go env GOROOT)/pkg/*/cmd
as the builders do when building binary releases.¹

For users who write programs that depend on go/types, it should be
reasonable to run the tests for go/types as part of 'go test all', and
those tests should pass even if they installed Go from a binary
release.

I had originally drafted this as a fallback to import from source only
if the affected packages can't be imported by the default export-data
importer. Unfortunately, I realized that we don't currently have a
builder that tests the actual release (#46900), so it is quite likely
that the fallback path would bit-rot and produce unexpected test
regressions.

So instead, we now unconditionally import from source in TestStdlib.
That makes the test substantially slower (~15s instead of ~5s on my
workstation), but with less risk of regression, and TestStdlib is
skipped in short mode already so short-mode test time is unaffected.

If we change the builders to test the actual release configuration, we
can consider restoring the faster path when export data is available.

¹https://github.com/golang/build/blob/df58bbac082bc87c4a3cdfe336d1ffe60bbaa916/cmd/release/release.go#L533-L545

For #43232

Change-Id: I764ec56926c104053bb2ef23cf258c8f0f773290
Reviewed-on: https://go-review.googlesource.com/c/go/+/330252


Trust: Bryan C. Mills <bcmills@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>