This mainly cleans up the few bits of code where we explicitly kept
support for Go 1.15.x. With v0.1.0 released, we can drop support now,
since the next v0.2.0 release will only support Go 1.16.x.
Also updates all modules, including test ones, to 'go 1.16'.
Note that the TOOLEXEC_IMPORTPATH refactor is not done here, despite all
the TODOs about doing so when we drop 1.15 support. This is because that
refactor needs to be done carefully and might have side effects, so it's
best to keep it to a separate commit.
Finally, update the deps.
In Go 1.15, if a dependency is required but not listed in go.mod/go.sum,
it's resolved and added automatically.
This is changing in 1.16. From that release, one will have to explicitly
update the mod files via 'go mod tidy' or 'go get'.
To get ahead of the curve, start using -mod=readonly to get the same
behavior in 1.15, and fix all existing tests.
The only tests that failed were imports.txt and syntax.txt, the only
ones to require other modules. But since we're here, let's add the 'go'
line to all go.mod files as well.
basic.txt just builds main.go without a module. Similarly, we leave
imports.txt without a GOPRIVATE, to test the 'go list -m' fallback.
For all other tests, explicitly set GOPRIVATE, to avoid two exec calls -
both 'go env GOPRIVATE' as well as 'go list -m'. Each of those calls
takes in the order of 10ms, so saving ~26 exec calls should easily add
to 200-300ms saved from 'go test -short'.
Error strings should never be capitalized.
A binsubstr line in one of the tests was duplicate and thus useless.
Remove duplicate or trailing spaces in test scripts.
Finally, add a TODO for an optimization I just spotted.
