Support indexing within the SwiftPM plugin sandbox (#66)

Resolves rdar://86786641.

## Summary

Resolves an issue when running DocC within the SwiftPM plugin sandbox
where LMDBs calls to `flock()`, which in turn call `fsctl()` are
blocked by the SwiftPM plugin sandbox.

## Details

Running `docc convert` with the `--index` flag from within a SwiftPM
plugin results in the following error:

```
[31m[1merror: [0m'docc convert' invocation failed with a nonzero exit code: '5'.
[plugin 'Swift-DocC Convert'] Swift/ErrorType.swift:200: Fatal error: Error raised at top level: PackagePlugin.PackageManagerProxyError.unspecified("ExitCode(rawValue: 1)")
[plugin 'Swift-DocC Convert'] 2021-12-17 10:47:27.747972-0800 Swift_DocC_Convert[93764:6330351] Swift/ErrorType.swift:200: Fatal error: Error raised at top level: PackagePlugin.PackageManagerProxyError.unspecified("ExitCode(rawValue: 1)")
error: plugin process ended by an uncaught signal: 5 <command: /usr/bin/sandbox-exec -p '(version 1)
(deny default)
(import "system.sb")
(allow file-read*)
(allow process*)
(allow file-write*
    (subpath "~/Developer/swift-docc-plugin/.build/plugins/outputs")
    (subpath "~/Developer/swift-docc-plugin/.build/plugins/cache")
    (subpath "/private/tmp")
    (subpath "/private/var/folders/8s/x4xvy8gd3m9c4r3zgdsqv9nh0000gn/T")
)
' ~/Developer/swift-docc-plugin/.build/plugins/cache/Swift_DocC_Convert>, <output:
'Swift/ErrorType.swift:200: Fatal error: Error raised at top level: PackagePlugin.PackageManagerProxyError.unspecified("ExitCode(rawValue: 1)")
2021-12-17 10:47:27.747972-0800 Swift_DocC_Convert[93764:6330351] Swift/ErrorType.swift:200: Fatal error: Error raised at top level: PackagePlugin.PackageManagerProxyError.unspecified("ExitCode(rawValue: 1)")
'>
```

Using the `fs_usage` tool, we determined that the sandbox is blocking
a call to `fsctl()` and throwing an error. `fsctl()` is used by the Darwin 
implementation of `flock()` which is used by LMDBs locking mechanism

However, it turns out we don't have a need
to run LMDB with locking here.

Per the LMDB docs:
> `MDB_NOLOCK:`
> Don't do any locking.
> If concurrent access is anticipated, the caller must
> manage all concurrency itself.

(https://github.com/LMDB/lmdb/blob/48a7fed59a8aae623deff415dda27097198ca0c1/libraries/liblmdb/lmdb.h#L619)

DocC is creating a new database here and will be the
only client to it while writing data-
so concurrent access across processes is not anticipated.

We already manage in-process access to the LMDB database
to ensure it's not concurrent.

Passing `.noLock` to the LMDB environment will
not regress DocC's existing experience and allows
DocC to run without issue in the SwiftPM sandbox.

## Testing

Existing tests continue to pass. An integration test that confirms
DocC is able to build a DocC archive that includes an index when run
as part of a SwiftPM plugin will be included in the Swift-DocC-Plugin
project and run as part of CI when the plugin is published.

Author: ethan-kusters

Sources/SwiftDocC/Indexing/Navigator/NavigatorIndex.swift

@@ -189,7 +189,7 @@ public class NavigatorIndex {
      */
     fileprivate init(withEmptyTree url: URL, bundleIdentifier: String) throws {
         self.url = url
-        self.environment = try LMDB.Environment(path: url.path, flags: [], maxDBs: 4, mapSize: 100 * 1024 * 1024) // mapSize = 100MB
+        self.environment = try LMDB.Environment(path: url.path, flags: [.noLock], maxDBs: 4, mapSize: 100 * 1024 * 1024) // mapSize = 100MB
         self.database = try environment.openDatabase(named: "index", flags: [.create])
         self.information = try environment.openDatabase(named: "information", flags: [.create])
         self.availability = try environment.openDatabase(named: "availability", flags: [.create])