gopls/internal/lsp/mod: simplify ModVulnerabilityDiagnostics
gopls/internal/lsp/mod: simplify ModVulnerabilityDiagnostics

Classify the unaffecting/affecting vulnerabilities as we
iterate over the vulnerabilities to generate info/warning
level diagnostics, instead of iterating over the list twice.

Change-Id: Icfac41c3897d443c368f0292279ca936daffe24e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/448978
Run-TryBot: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools