…ge requirements consistently

SetRequireSeparateIndirect now makes a stronger attempt to keep
automatically added requirements in two blocks: one containing only
direct requirements and one containing only indirect
requirements. SetRequireSeparateIndirect will find or add these two
blocks (commented blocks are left alone). New requirements are added
to one of these two blocks. Existing requirements may be moved between
these two blocks if their markings change.

SetRequireSeparateIndirect attempts to preserve existing structure in
the file by not adding requirements to or moving requirements from
blocks with block-level comments and blocks other than the last
uncommented direct-only and indirect-only block.

As an exception to aid with migration, if the file contains a single
uncommented block of requirements (as would the be the case if the
user started with a 1.16 go.mod file, changed the go directive to
1.17, then ran 'go mod tidy'), SetRequireSeparateIndirect will split
the block into direct-only and indirect-only blocks.

This is a change in behavior, but it has no semantic effect, and it
should result in cleaner, more stable go.mod files.

For golang/go#47756

Change-Id: Ifa20bb084c6bdaf1e00140600380857de8afa320
Reviewed-on: https://go-review.googlesource.com/c/mod/+/343431
Trust: Jay Conrod <jayconrod@google.com>
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
(cherry picked from commit 4be982b)
Reviewed-on: https://go-review.googlesource.com/c/mod/+/351317

…rt lines to blocks

When reading go.mod, SetRequireSeparateIndirect will insert new
requirements into the last uncommented direct-only or indirect-only
block OR line. If the last such statement is a line,
SetRequireSeparateIndirect converts it to a block before inserting new
requirements. Cleanup will convert it back to a line later if no
requirements are inserted.

For golang/go#47756

Change-Id: Id8ee3b0ce2d005488ddb3d9a5349115bd93938e7
Reviewed-on: https://go-review.googlesource.com/c/mod/+/348576
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
(cherry picked from commit 37dd689)
Reviewed-on: https://go-review.googlesource.com/c/mod/+/351318

…tently for go 1.17+

If go.mod declares 1.17 or higher, when the go command rewrites go.mod
(for example, after 'go mod tidy'), it will be more consistent about
moving requirements in two blocks, one containing only direct
requirements, and one containing only indirect requirements.

The go command will not move requirements into or out of a block with
comments. It may still update versions and "// indirect" comments, and
it may delete unneeded requirements though.

Fixes #47756

Change-Id: Ia6fb3e302be53097893abf01aa7cea60ac7b069a
Reviewed-on: https://go-review.googlesource.com/c/go/+/343432
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit 3c764ba)
Reviewed-on: https://go-review.googlesource.com/c/go/+/351465
TryBot-Result: Go Bot <gobot@golang.org>