internal/lsp/source: use 'gopls.' instead of 'gopls_'

And, export CommandPrefix const so api_json generator can use it.

Update golang/go#41187

Change-Id: Ie976b774d64ba707152b6f659a15aef19ef12dcd
Reviewed-on: https://go-review.googlesource.com/c/tools/+/263057
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Peter Weinberger <pjw@google.com>

gopls/doc/commands.md

@@ -4,75 +4,75 @@ This document describes the LSP-level commands supported by `gopls`. They cannot
 
 <!-- BEGIN Commands: DO NOT MANUALLY EDIT THIS SECTION -->
 ### **Run go generate**
-Identifier: `gopls_generate`
+Identifier: `gopls.generate`
 
 generate runs `go generate` for a given directory.
 
 
 ### **Fill struct**
-Identifier: `gopls_fill_struct`
+Identifier: `gopls.fill_struct`
 
 fill_struct is a gopls command to fill a struct with default
 values.
 
 
 ### **Regenerate cgo**
-Identifier: `gopls_regenerate_cgo`
+Identifier: `gopls.regenerate_cgo`
 
 regenerate_cgo regenerates cgo definitions.
 
 
 ### **Run test(s)**
-Identifier: `gopls_test`
+Identifier: `gopls.test`
 
 test runs `go test` for a specific test function.
 
 
 ### **Run go mod tidy**
-Identifier: `gopls_tidy`
+Identifier: `gopls.tidy`
 
 tidy runs `go mod tidy` for a module.
 
 
 ### **Undeclared name**
-Identifier: `gopls_undeclared_name`
+Identifier: `gopls.undeclared_name`
 
 undeclared_name adds a variable declaration for an undeclared
 name.
 
 
 ### **Upgrade dependency**
-Identifier: `gopls_upgrade_dependency`
+Identifier: `gopls.upgrade_dependency`
 
 upgrade_dependency upgrades a dependency.
 
 
 ### **Run go mod vendor**
-Identifier: `gopls_vendor`
+Identifier: `gopls.vendor`
 
 vendor runs `go mod vendor` for a module.
 
 
 ### **Extract to variable**
-Identifier: `gopls_extract_variable`
+Identifier: `gopls.extract_variable`
 
 extract_variable extracts an expression to a variable.
 
 
 ### **Extract to function**
-Identifier: `gopls_extract_function`
+Identifier: `gopls.extract_function`
 
 extract_function extracts statements to a function.
 
 
 ### **Toggle gc_details**
-Identifier: `gopls_gc_details`
+Identifier: `gopls.gc_details`
 
 gc_details controls calculation of gc annotations.
 
 
 ### **Generate gopls.mod**
-Identifier: `gopls_generate_gopls_mod`
+Identifier: `gopls.generate_gopls_mod`
 
 generate_gopls_mod (re)generates the gopls.mod file.