PEP 751: add editable to packages.directory (#3888)

* PEP 751: add `editable` to `packages.directory`
Along the way address some other comments.

peps/pep-0751.rst

@@ -46,9 +46,17 @@ what would be installed into an environment ahead of time.
 
 The lack of a standard also has some drawbacks. For instance, any tooling that
 wants to work with lock files must choose which format to support, potentially
-leaving users unsupported (e.g., if Dependabot_ chose not to support PDM,
-support by cloud providers who can do dependency installations on your behalf,
-etc.).
+leaving users unsupported (e.g., Dependabot_ only supporting select tools,
+same for cloud providers who can do dependency installations on your behalf,
+etc.). It also impacts portability between tools, which causes vendor lock-in.
+By not having compatibility and interoperability it fractures tooling around
+lock files where both users and tools have to choose what lock file format to
+use upfront and making it costly to use/switch to other formats. Rallying
+around a single format removes that cost/barrier.
+
+.. note::
+
+   Much of the motivation from :pep:`665` also applies to this PEP.
 
 
 =========
@@ -424,7 +432,7 @@ unless otherwise specified for the same reason.
 ``[[packages.files]]``
 ----------------------
 
-- Must be specified if ``[packages.vcs]`` and ``packages.directory`` is not
+- Must be specified if ``[packages.vcs]`` and ``[packages.directory]`` is not
   (although may be specified simultaneously with the other options).
 - Array of tables
 - Tables can be written inline.
@@ -470,10 +478,12 @@ unless otherwise specified for the same reason.
 - Optional
 - String
 - URI where the file was found when the lock file was generated.
+- If the URI is a relative file path, it is considered relative to the lock
+  file.
 - Useful for documenting where the file was originally found and potentially
-  where to look for the file if not already downloaded/available.
-- Installers MUST NOT assume the URL will always work, but installers MAY use
-  the URL if it happens to work.
+  where to look for the file if it is not already downloaded/available.
+- Installers MUST NOT assume the URI will always work, but installers MAY use
+  the URI if it happens to work.
 
 
 ``packages.files.hash``
@@ -489,8 +499,8 @@ unless otherwise specified for the same reason.
 ``[packages.vcs]``
 ------------------
 
-- Must be specified if ``[[packages.files]]`` and ``packages.directory`` is not
-  (although may be specified simultaneously with the other options).
+- Must be specified if ``[[packages.files]]`` and ``[packages.directory]`` is
+  not (although may be specified simultaneously with the other options).
 - Table representing the version control system containing the package and
   version.
 
@@ -534,16 +544,30 @@ unless otherwise specified for the same reason.
   version.
 
 
-``packages.directory``
-----------------------
+``[packages.directory]``
+------------------------
 
-- Must be specified if ``[[packages.files]]`` and ``packages.vcs`` is not
+- Must be specified if ``[[packages.files]]`` and ``[packages.vcs]`` is not
   and doing per-package locking.
+- Table representing a source tree found on the local file system.
+
+
+``packages.directory.path``
+'''''''''''''''''''''''''''
+
 - String
 - A local directory where a source tree for the package and version exists.
-- Not valid under ``[[file-locks]]`` as this PEP does not make an attempt to
-  specify a mechanism for verifying file contents have not changed since locking
-  was performed.
+- The path MUST use forward slashes as the path separator.
+- If the path is relative it is relative to the location of the lock file.
+
+
+``packages.directory.editable``
+'''''''''''''''''''''''''''''''
+
+- Boolean
+- Optional (defaults to ``false``)
+- Flag representing whether the source tree should be installed as an editable
+  install.
 
 
 ``[[packages.build-requires]]``