Remove vendor directory.

This was something previously brought up in tektoncd#1607, but was deferred to
allow for continued discussion.

Pros of making this change:
* Reduces size of incoming PRs when new dependencies are used.
* Removes confusion around source of truth for dependencies (e.g. vendor
vs mod). This was an issue that tektoncd#1763 solved by pinning to vendor, this
will solve the same problem differently by always deferring to the
version in go.mod.
* Allows easier use of upstream and dependent types without the use of
vendor - see
https://gist.github.com/wlynch/325293bc3fbc488d3b3d319f3a93bbea for an
example of why this is difficult today.

Risks of making this change:
* Dependencies will not be present in initial clone. They will be
fetched dynamicly when needed, which may cause workflow distruptions.
* This breaks compatibility with older versions of Go (though for
certain projects like triggers, we already require Go >= 1.13).

Third-party obligations (for things like making source available in
images) have already been addressed in tektoncd#1607.

As with tektoncd#1607, this change is more easily viewed by running `git diff
master -- ':!third_party' ':!vendor'`

hack/update-codegen.sh

@@ -18,7 +18,7 @@ set -o errexit
 set -o nounset
 set -o pipefail
 
-source $(dirname $0)/../vendor/github.com/tektoncd/plumbing/scripts/library.sh
+source $(go list -f '{{.Dir}}' github.com/tektoncd/plumbing)/scripts/library.sh
 
 # generate the code with:
 # --output-base    because this script should also be able to run inside the vendor dir of

hack/update-deps.sh

@@ -18,12 +18,8 @@ set -o errexit
 set -o nounset
 set -o pipefail
 
-source $(dirname $0)/../vendor/github.com/tektoncd/plumbing/scripts/library.sh
+source $(go list -f '{{.Dir}}' github.com/tektoncd/plumbing)/scripts/library.sh
 
 cd ${REPO_ROOT_DIR}
 
-# Prune modules.
-go mod tidy
-go mod vendor
-
 update_licenses third_party/

hack/verify-codegen.sh

@@ -18,7 +18,7 @@ set -o errexit
 set -o nounset
 set -o pipefail
 
-source $(dirname $0)/../vendor/github.com/tektoncd/plumbing/scripts/library.sh
+source $(go list -f '{{.Dir}}' github.com/tektoncd/plumbing)/scripts/library.sh
 
 readonly TMP_DIFFROOT="$(mktemp -d ${REPO_ROOT_DIR}/tmpdiffroot.XXXXXX)"
 

test/e2e-common.sh

@@ -16,8 +16,7 @@
 
 # Helper functions for E2E tests.
 
-go get -d github.com/tektoncd/plumbing
-source $(go list -m -f '{{.Dir}}' github.com/tektoncd/plumbing)/scripts/e2e-tests.sh
+source $(go list -f '{{.Dir}}' github.com/tektoncd/plumbing)/scripts/e2e-tests.sh
 
 function teardown() {
     subheader "Tearing down Tekton Pipelines"

test/presubmit-tests.sh

@@ -26,8 +26,7 @@
 export DISABLE_MD_LINTING=1
 export DISABLE_MD_LINK_CHECK=1
 
-go get -d github.com/tektoncd/plumbing
-source $(go list -m -f '{{.Dir}}' github.com/tektoncd/plumbing)/scripts/presubmit-tests.sh
+source $(go list -f '{{.Dir}}' github.com/tektoncd/plumbing)/scripts/presubmit-tests.sh
 
 function post_build_tests() {
   header "running golangci-lint"