internal/lsp/source: simplify workspace symbol package collection
internal/lsp/source: simplify workspace symbol package collection

While evaluating workspace symbols, we avoid duplicate packages per
package path in an effort to save computing time and eliminate duplicate
results. But for simplicity, we later guard to ensure that we don't ever
walk files twice.

We can just rely on this guard to prevent duplication, and walk all
known packages. This ensures we don't miss symbols, at minimal
additional cost, and simplies the code.

Fixes golang/go#42791

Change-Id: I97000dbe29e67bda6071d5119a6edb7c5fa4da80
Reviewed-on: https://go-review.googlesource.com/c/tools/+/272686
Run-TryBot: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Trust: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Reviewed-by: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>

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