Only add project-metadata source of type "project" when present

[jromero]

Summary

pack will no longer produce project metadata source of type project when project descriptor is not present.

Additionally, this PR makes this feature as a whole experimental. Primarily because we are still missing the other types per #1139.

Output

Before

❯ ls ~/dev/buildpacks/samples/apps/bash-script/
README.md             app.sh                bash-script-buildpack

❯ pack-dev build -B cnbs/sample-builder:bionic -p ~/dev/buildpacks/samples/apps/bash-script/ bash-app
...

❯ docker inspect bash-app | jq -r
[
  {
...
     "Labels": {
        "io.buildpacks.project.metadata": "{\"source\":{\"type\":\"project\",\"version\":{\"declared\":\"\"},\"metadata\":{\"url\":\"\"}}}",
...

After

❯ ls ~/dev/buildpacks/samples/apps/bash-script/
README.md             app.sh                bash-script-buildpack

❯ pack-dev build -B cnbs/sample-builder:bionic -p ~/dev/buildpacks/samples/apps/bash-script/ bash-app
...

❯ docker inspect bash-app | jq -r
[
  {
...
     "Labels": {
        "io.buildpacks.project.metadata": "{}",
...

❯ cat ~/dev/buildpacks/samples/apps/bash-script/project.toml
[project]
id = "io.buildpacks.bash-script"
name = "Bash Script"
version = "1.0.0"

[build]
exclude = [
    "README.md",
    "bash-script-buildpack"
]

[[build.buildpacks]]
uri = "bash-script-buildpack/"


❯ pack-dev build -B cnbs/sample-builder:bionic -p ~/dev/buildpacks/samples/apps/bash-script/ bash-app
...

❯ docker inspect bash-app | jq -r
[
  {
...
     "Labels": {
        "io.buildpacks.project.metadata": "{\"source\":{\"type\":\"project\",\"version\":{\"declared\":\"1.0.0\"},\"metadata\":{\"url\":\"\"}}}",
...

Documentation

• Should this change be documented?
  • Yes, see #___
  • No

Related

Related to #1138

[codecov]

Codecov Report

Merging #1285 (38d6ad1) into release/0.21.0 (a7487c9) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@                Coverage Diff                 @@
##           release/0.21.0    #1285      +/-   ##
==================================================
+ Coverage           81.08%   81.09%   +0.01%     
==================================================
  Files                 143      143              
  Lines                8757     8761       +4     
==================================================
+ Hits                 7100     7104       +4     
  Misses               1218     1218              
  Partials              439      439              

Flag	Coverage Δ
os_linux	79.71% <100.00%> (+0.01%)	⬆️
os_macos	76.93% <100.00%> (-0.02%)	⬇️
os_windows	80.99% <100.00%> (+0.01%)	⬆️
unit	81.09% <100.00%> (+0.01%)	⬆️

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