cmd/doc: fix "builtin" package parsing
cmd/doc: fix "builtin" package parsing

As stated in the code, "The builtin package needs special treatment: its
symbols are lower case but we want to see them, always". Thus, cmd/doc
forces the -u flag if the package being queried is called "builtin".
However, this happens after having already parsed the package. This
CL forces the -u flag just after parsing the command arguments and
before parsing any package.

Fixes #49796.

Change-Id: If690a900c7cfd1700feecb9529bd4344c3c249d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/367134
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>
Run-TryBot: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Trust: Emmanuel Odeke <bd234ba4276433f0e5fc7a8fa2d18274fa711567@orijtech.com>

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