Go 1.21.0 was released in August 2023, so our upcoming release
will no longer support the Go 1.20 release series.
The first Go 1.22 release candidate is also due in December 2023,
less than a month from now, so dropping 1.20 will simplify 1.22 work.
First, teach scripts/gen-go-std-tables.sh to omit test packages,
since runtime/metrics_test would always result in an error.
Instead, make transformLinkname explicitly skip that package,
leaving a comment about a potential improvement if needed.
Second, the only remaining "not found" error we had was "maps" on 1.20,
so rewrite that check based on ImportPath and GoVersionSemver.
Third, detect packages with the "exclude all Go files" error
by looking at CompiledGoFiles and IgnoredGoFiles, which is less brittle.
This means that we are no longer doing any filtering on pkg.Error.Err,
which means we are less likely to break with Go error message changes.
Fourth, the check on pkg.Incomplete is now obsolete given the above,
meaning that the CompiledGoFiles length check is plenty.
Finally, stop trying to be clever about how we print errors.
Now that we're no longer skipping packages based on pkg.Error values,
printing pkg.DepsErrors was causing duplicate messages in the output.
Simply print pkg.Error values with only minimal tweaks:
including the position if there is any, and avoiding double newlines.
Overall, this makes our logic a lot less complicated,
and garble still works the way we want it to.
The first makes our test scripts more consistent, as all external
program executions happen via "exec" and are not as easily confused
with custom builtin commands like our "generate-literals".
The second catches mistakes if any of our txtar files have duplicate
files, where all but one of the contents would be ignored before.
I mistakenly understood that, when the DepsErrors field has errors,
the Error field would contain an error as well.
That is not always the case; for example,
the imports_missing package in the added test script
had DepsErrors set but Error empty, causing a nil dereference panic.
Make the code more robust, and report both kinds of load errors.
Fixes#694.
Daniel Martí