internal/testenv: avoid rebuilding all of std in WriteImportcfg
Instead, have the caller pass in an explicit list of the packages
(if any) they need.
After #47257, a builder running a test does not necessarily have the
entire standard library already cached, especially when running tests
in sharded mode. testenv.WriteImportcfg used to write an importcfg for
the entire standard library — which required rebuilding the entire
standard library — even though most tests need only a tiny subset.
This reduces the time to test internal/abi with a cold build cache on
my workstation from ~16s to ~0.05s.
It somewhat increases the time for 'go test go/internal/gcimporter'
with a cold cache, from ~43s to ~54s, presumably due to decreased
parallelism in rebuilding the standard library and increased overhead
in re-resolving the import map. However, 'go test -short' running time
remains stable (~5.5s before and after).