[Bug go/65462] Use of 'go get' with gccgo is not finding dependencies correctly

["boger at us dot ibm.com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65462

--- Comment #1 from boger at us dot ibm.com ---
Created attachment 35077
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35077&action=edit
Fix dependencies in go tool for gccgo

The code to determine when to import packages was not quite catching the cases
it should.  It really needs to know what packages are part of the GO standard
packages and not try to load those -- if they aren't then they should be
loaded.

This makes a few changes to the previous:
- change the name of reqPkgSrc to reqStdPkgSrc to indicate when we should
require the source for GO standard packages when loading packages.
- generate the list of standard packages found in libgo and make that available
to build into the go tool
- set the flag Standard in the Package structure correctly for gccgo, based on
the list of expected libgo standard packages
- use the reqStdPkgSrc flag and the package.Standard flag to determine if the
load of the package should be attempted