internal/lsp/source: don't build low scoring workspace symbols
internal/lsp/source: don't build low scoring workspace symbols

By refactoring the symbol resolution to defer symbol construction until
after scoring, we can avoid some unnecessary string allocation.

Benchmark (fastfuzzy "test" in x/tools): 21ms->17ms
Benchmark (fuzzy "test" in x/tools): 46ms->42ms
Benchmark (fastfuzzy "test" in kubernetes): 199ms->183ms

Change-Id: I9de72eb203c9971acc1afe89657976ce193b5a5d
Reviewed-on: https://go-review.googlesource.com/c/tools/+/338696
Trust: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Run-TryBot: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>

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