internal/lsp/cache: simplify workspace construction
internal/lsp/cache: simplify workspace construction

Remove some duplication in the construction of cache.workspace
instances.

For golang/go#44696

Change-Id: Ic955fa3f9a574781f627c550694f0b87ad2013e1
Reviewed-on: https://go-review.googlesource.com/c/tools/+/382114
Trust: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Run-TryBot: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Reviewed-by: Michael Matloob <4562438b12c14e81a47550acde616ff328a909f8@golang.org>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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