Users working in large repositories may want to include only selected
directories in their workspace to avoid memory usage and performance
slowdowns. Add support for inclusion/exclusion filters that control what
directories are searched for workspace packages and modules. Packages
that are excluded by the filter may still be loaded as non-workspace
packages if other things depend on them.

For a description of the option's syntax, see the documentation.

Note that because we don't have any way to communicate the filters to
packages.Load, we still run go list on the unfiltered workspace scope,
then throw away the irrelevant packages. That may cost us, especially in
workspaces with many files.

Comments on the naming welcome. Also, if you know any places I may have
missed applying the filter, please do tell. One thing I thought of is
file watching, but that's covered because allKnownSubdirs works off of
workspace files and those are already filtered.

Possible enhancements:
 - Support glob patterns.
 - Apply filters during the goimports scan.
 - Figure out how to apply the filters to packages.Load. I don't know
 how to do it while still being build system neutral though.

Closes golang/go#42473, assuming none of the enhancements are required.

Change-Id: I9006a7a361dc3bb3c11f78b05ff84981813035a0
Reviewed-on: https://go-review.googlesource.com/c/tools/+/275253
Trust: Heschi Kreinick <heschi@google.com>
Run-TryBot: Heschi Kreinick <heschi@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>