Merge extras in lockfile (#5181)
As user, you specify a list of extras. Internally, we decompose this
into one virtual package per extra. We currently leak this abstraction
by writing one entry per extra to the lockfile:
```toml
[[distribution]]
name = "foo"
version = "4.39.0.dev0"
source = { editable = "." }
dependencies = [
{ name = "pandas" },
{ name = "pandas", extra = "excel" },
{ name = "pandas", extra = "hdf5" },
{ name = "pandas", extra = "html", marker = "os_name != 'posix'" },
{ name = "pandas", extra = "output-formatting", marker = "os_name == 'posix'" },
{ name = "pandas", extra = "plot", marker = "os_name == 'posix'" },
]
```
Instead, we should merge the extras into a list of extras, creating a
more concise lockfile:
```toml
[[distribution]]
name = "foo"
version = "4.39.0.dev0"
source = { editable = "." }
dependencies = [
{ name = "pandas", extra = ["excel", "hdf5"] },
{ name = "pandas", extra = ["html"], marker = "os_name != 'posix'" },
{ name = "pandas", extra = ["output-formatting", "plot"], marker = "os_name == 'posix'" },
]
```
The base package is now implicitly included, as it is in PEP 508.
Fixes #4888
