Rename Go module path from github.com/rcowham/p4lf to workshop.perforce.com/p4lf.
The rcowham prefix was a carryover from early research into go-libtail
(which is hosted at github.com/rcowham/go-libtail). p4lf is hosted on
the Perforce Public Depot / Workshop, not GitHub, so workshop.perforce.com
is the correct canonical module path.
Updated: go.mod, all *.go files with internal imports, Makefile (MODULE
and LDFLAGS). Session logs retain the old path as accurate historical
record.