internal/importpath: extract shared import path validation logic

[eaudetcobello]

Extract the duplicated import path validation logic
I created a new package: internal/importpath.

I've only refactored one call site so reviewers can validate my approach
and package placement before I proceed with the remaining sites.
I've placed it under internal/ but that doesn't seem correct.
The code is failing the TestDependencies test.

The remaining call sites are: go/build/read.go, go/printer/nodes.go,
go/types/resolver.go, cmd/go/internal/load/pkg.go,
cmd/compile/internal/types2/resolver.go.

Updates #74446

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Patch Set 1:

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