dev: add GL_DEBUG=govet to see enabled analyzers

[alexandear]

The PR adds the debug key govet that can be used to see default, all available, and enabled govet analyzers:

GL_DEBUG=govet ./golangci-lint run --enable=govet

Example of debug output:

❯ GL_DEBUG=govet ./golangci-lint run --enable=govet
DEBU [govet] All available analyzers (41): [appends asmdecl assign atomic atomicalign bools buildtag cgocall composites copylocks deepequalerrors defers directive errorsas fieldalignment findcall framepointer httpresponse ifaceassert loopclosure lostcancel nilfunc nilness printf reflectvaluecompare shadow shift sigchanyzer slog sortslice stdmethods stringintconv structtag testinggoroutine tests timeformat unmarshal unreachable unsafeptr unusedresult unusedwrite] 
DEBU [govet] Default analyzers (32): [appends asmdecl assign atomic bools buildtag cgocall composites copylocks defers directive errorsas framepointer httpresponse ifaceassert loopclosure lostcancel nilfunc printf shift sigchanyzer slog stdmethods stringintconv structtag testinggoroutine tests timeformat unmarshal unreachable unsafeptr unusedresult] 
DEBU [govet] Enabled by config analyzers (35): [appends asmdecl atomicalign bools buildtag cgocall composites copylocks deepequalerrors defers directive errorsas fieldalignment findcall framepointer httpresponse ifaceassert loopclosure lostcancel nilfunc printf reflectvaluecompare shift sigchanyzer slog sortslice stdmethods stringintconv structtag testinggoroutine tests timeformat unmarshal unreachable unsafeptr] 

The implementation of govet output debugging is similar to the existing one for gocritic checks.

Previously, the documentation suggested using go tool vet help to see all analyzers. However, this might be inaccurate as we could forget to update govet.allAnalyzers and govet.defaultAnalyzers slices. To avoid this, I changed the documentation to GL_DEBUG=govet golangci-lint run --enable=govet.

[ldez]

LGTM