gopls/internal/vulncheck: log progress
gopls/internal/vulncheck: log progress

Log before/after package loading and govulncheck.Source calls.
All stderr output of `gopls vulncheck` command is forwarded to the
client as LSP progress reports and visible to users.

go/packages.Load can be traced further if GOPACKAGESDEBUG env var
is set. Instrumentation in govulncheck.Source is still under
discussion in upstream.

For golang/go#56795

Change-Id: I8244930494aed17b319887bf96c2523f3215fa50
Reviewed-on: https://go-review.googlesource.com/c/tools/+/452055
Run-TryBot: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Suzy Mueller <a1fd7fee333bfb1457f0c89ab258f70a76be3c5d@golang.org>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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