cmd/go: fix recognition of Fossil checkout directories

Use ".fslckout" or "_FOSSIL_" files to correctly identify the root of
Fossil checkout directories.

Previously, Go has assumed VCS checkouts are indicated by a dot-file
named after the VCS command (eg, .git, .hg, .fossil). This is not
normally true for Fossil, but has worked so far since Go downloads
repositories into a ".fossil" file before opening (checking out).

Using the incorrect path prevents Go identifying Fossil checkouts and
correctly tagging with -buildvcs (to be added in a follow up CL).

Change-Id: I3c030a0b600dfe7f4b3fcf1551b1d7cb84629b81
Reviewed-on: https://go-review.googlesource.com/c/go/+/357954


Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>