refactor(pnpm): move pnpm parsing logic to pnpm.bzl

[jbedard]

Is this worth loosing the git blame history on all this code? The utils.bzl is getting getting pretty big and I keep thinking this stuff should live on its own...

Some of the moved code was reordered to group things by pnpm lockfile version a bit better, I think I added _v5 to one method name to highlight that it's only for v5 lockfiles, otherwise the moved code is unchanged.

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Changes are visible to end-users:no

Test plan

• Covered by existing test cases

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e2e/package_json_module

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