Now that we don't need to support git@
pseudo-urls, we can simplify the test for
valid VCS URLs based on link.is_vcs, which
is turns is based on the URL scheme.

This also means we fail earlier if a git@ pseudo
URL is used.

Since VCS requirements are not validated to be URLs
in Requirement constructors, we can simplify update_editable.

Vendoring changes for release 21.0 (excluding setuptools)

Remove support for legacy cache entries

Use Formatter.default_time_format after dropping PY2

Avoid downloading multiple candidates of a same version

Revert #7969 and fix VCS stdout/stderr capture

Remove unused wheel_cache argument in freeze operation

Remove deprecation deadline for #8368

Use dict and set comprehension where available

Prefer subprocess.DEVNULL over open(os.devnull, 'w')

Since 41a30089, Candidate objects prepare their underlying
distribution eagerly on creation, so the error can be caught
deterministically to trigger backtracking.

This however has a negative side-effect. Since dist preparation involves
metadata validation, a remote distribution must be downloaded on
Candidate creation. This means that an sdist will be downloaded,
validated, and discarded (since its version is known to be incompatible)
during backtracking.

This commit moves version deduplication of candidates from indexes to
before the Candidate object is created, to avoid unneeded preparation.

Note that we still need another round of deduplication in FoundCandidates
to remove duplicated candidates when a distribution is already installed.

Verify built wheel contains valid metadata

Make get_similar_command() return None for no value

Remove unused utility function consume()

Unused since e706af20.

Follows the more conventional pattern of using None, instead of False,
when no value can be returned. This simplifies typing a bit by using
Optional instead of Union[bool, ...].

Remove unnecessary __ne__ definitions

Harmonize return type of VersionControl.get_revision in subclasses

Simplify check_binary_allowed

Handle ._get_svn_url_rev() returning None in .get_remote_url()

The method Subversion._get_svn_url_rev() will sometimes return None for
a remote URL. The calling code should handle this. If it is None, raise
a RemoteNotFoundError as prescribed by the parent class docstring.

Followup to 0b761a16.

check_binary_allowed is only used to check
if a wheel needs to be built in 'pip install' mode.
It mixed format control and pep517 mode check.

We change it so it checks allowed formats only,
which leads to better readability of _should_build().

Previously, the Subversion subclass violated the parent's type signature
by returning an int, but it is now coerced to a str to match the
expected signature.

Unnecessary since dropping Python 2 support. In Python 3, `__ne__`
defaults to the opposite of of `__eq__`.

https://docs.python.org/3/reference/datamodel.html#object.__ne__

> For `__ne__()`, by default it delegates to `__eq__()` and inverts the
> result unless it is `NotImplemented`.

Replace compat.expanduser() with os.path.expanduser()

The upstream bug has been fixed and released in all supported Python
version: https://bugs.python.org/issue14768

Remove unnecessary "noqa" comments

Remove unused class CopytreeKwargs

Remove unused type VersionCandidate