cmd/go/internal/modload: track which packages are in 'all' during loading

If the user requests the 'all' pattern in addition to explicit roots
outside of 'all', we should not load the transitive dependencies of
those explicit roots as if they were *in* 'all'. Without the '-test'
flag, we should not load test dependencies of any package outside of
'all'. Even *with* the '-test' flag, we should only load test
dependencies of the requested roots, not test dependencies of other
packages that happen to be imported by those roots.

More precise tracking of membership in 'all' will be important when we
implement lazy loading, because membership in 'all' determines which
module dependencies we will record in the main module's go.mod file.

This change also reduces reliance on global state, factors out the
loading process into several smaller functions, and sets us up to
reuse the 'go mod vendor' version of the 'all' pattern for lazy
loading.

For #36460
Fixes #40799

Change-Id: I5ca21c86a860daee1316f732cea131a331d8ddf9
Reviewed-on: https://go-review.googlesource.com/c/go/+/240505
Run-TryBot: Bryan C. Mills <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Jay Conrod <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>

Author: Bryan C. Mills