cmd/go: report an added go directive as added, not downgraded

[harjothkhara]

When 'go get go@version' runs in a module whose go.mod has no go
directive, the go command synthesizes the current version into the
in-memory requirements before writing the file. reportChanges then
compared the requested version against that synthesized version and
printed a misleading message, for example:

go: downgraded go 1.27 => 1.20

even though the module never declared go 1.27. Report the change as the
directive being added instead:

go: added go 1.20

Determine whether the go.mod originally declared a go directive from the
main module's index, which is not affected by the synthesized directive,
and treat an absent directive as having no previous version. Existing go
directives are still reported as upgrades or downgrades, and changes that
only write back the synthesized version stay silent.

This affects 'go get' in module mode with a single main module. In
workspace mode the change report is still suppressed for an added go
directive, but that is a separate, pre-existing issue (the workspace
load synthesizes the directive into the in-memory go.mod before the
change is computed) and is out of scope here.

Fixes #63507
Fixes #70090

[JoesSon72]

Thanks Jesus Loves you #SayerTurner

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Message from Harjoth Khara:

Patch Set 3:

(1 comment)

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Message from Dmitri Shuralyov:

Patch Set 3: Code-Review+1

(1 comment)

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Message from Harjoth Khara:

Patch Set 4:

(2 comments)

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Message from Harjoth Khara:

Patch Set 4:

(1 comment)

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