pin: no op when local vcs directory does not contain opam file

[rjbou]

opam file is retrieved from local directory even if it is vcs url, not from the branch specified in the given url.

### mkdir pin-empty
### git -C pin-empty init -q --initial-branch=master
### git -C pin-empty config core.autocrlf false
### opam pin --no-action ./pin-empty
### <pin-empty/file>
content
### git -C pin-empty add -A
### git -C pin-empty commit -qm "init"
### git -C pin-empty branch another
### <pin:pin-empty/opam>
opam-version: "2.0"
### git -C pin-empty add -A
### git -C pin-empty commit -qm 'opam'
### git -C pin-empty checkout another
Switched to branch 'another'
### ls pin-empty
file
### OPAMDEBUG=-1
### opam pin --no-action ./pin-empty
CLI                             Parsing CLI version 2.2
GSTATE                          LOAD-GLOBAL-STATE @ ${BASEDIR}/OPAM
RSTATE                          LOAD-REPOSITORY-STATE @ ${BASEDIR}/OPAM
CACHE(repository)               Loaded ${BASEDIR}/OPAM/repo/state-003ACE73.cache in 0.000s
RSTATE                          Cache found
STATE                           LOAD-SWITCH-STATE @ pinning
CACHE(installed)                Loaded ${BASEDIR}/OPAM/pinning/.opam-switch/packages/cache in 0.000s
STATE                           Inferred invariant: from base packages {}, (roots {}) => []
FILE(switch-config)             Wrote ${BASEDIR}/OPAM/pinning/.opam-switch/switch-config in 0.000s
STATE                           Switch state loaded in 0.000s
FILE(switch-state)              Wrote ${BASEDIR}/OPAM/pinning/.opam-switch/backup/state-20230215111756.export in 0.000s
PARALLEL                        Iterate over 0 task(s) with 1 process(es)
SYSTEM                          rm ${BASEDIR}/OPAM/pinning/.opam-switch/backup/state-20230215111756.export
### opam pin --no-action ./pin-empty#inexistant
CLI                             Parsing CLI version 2.2
GSTATE                          LOAD-GLOBAL-STATE @ ${BASEDIR}/OPAM
RSTATE                          LOAD-REPOSITORY-STATE @ ${BASEDIR}/OPAM
CACHE(repository)               Loaded ${BASEDIR}/OPAM/repo/state-003ACE73.cache in 0.000s
RSTATE                          Cache found
STATE                           LOAD-SWITCH-STATE @ pinning
CACHE(installed)                Loaded ${BASEDIR}/OPAM/pinning/.opam-switch/packages/cache in 0.000s
STATE                           Inferred invariant: from base packages {}, (roots {}) => []
FILE(switch-config)             Wrote ${BASEDIR}/OPAM/pinning/.opam-switch/switch-config in 0.000s
STATE                           Switch state loaded in 0.000s
FILE(switch-state)              Wrote ${BASEDIR}/OPAM/pinning/.opam-switch/backup/state-20230215111756.export in 0.000s
PARALLEL                        Iterate over 0 task(s) with 1 process(es)
SYSTEM                          rm ${BASEDIR}/OPAM/pinning/.opam-switch/backup/state-20230215111756.export