internal/imports: read entire API dir in mkstdlib.go
internal/imports: read entire API dir in mkstdlib.go

Read the entire API directory, so that we don't need to edit mkstdlib.go
when regenerating zstdlib.go for new go versions.

Joint with Dylan Le.

Co-Authored-By: 9147415174ef368c95d39c7c9668a6ab201fcffd@u.rochester.edu.
Change-Id: If5c71bfcfd2104f923df10cf21d16b5280fed025
Reviewed-on: https://go-review.googlesource.com/c/tools/+/438504
Reviewed-by: Dylan Le <19fed614f843e0de8399ba7a25e061ee66748da3@google.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

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