Cosmetic issue when reporting failed ncurses detection

Cosmetic issue when reporting failed ncurses detection

```
Found pkg-config: /usr/bin/pkg-config (0.29.1)
ncursesw6-config found: NO
Run-time dependency curses found: NO (tried pkgconfig, pkgconfig, pkgconfig, pkgconfig, config-tool and system)

meson.build:2:0: ERROR: Dependency "curses" not found, tried pkgconfig, pkgconfig, pkgconfig, pkgconfig, config-tool and system
```

Note the repeated `pkgconfig` in tried methods.

**To Reproduce**

```
project('test'', 'c')
dependency('curses')
```

without any curses implemenation installed.

**Expected behavior**

`pkgconfig` shouldn't be meaninglessly repeated? either qualify it with the actual .pc name, or elide the repetitions?

**system parameters**
* native
* Ubuntu 20.04
* Python 3.8.2
* meson current master

Reproduce the issue with the shown project and dependency('curses') call on a system without a curses implementation. Start at Meson's dependency detection and diagnostic reporting for pkg-config, then verify that the failure message no longer repeats `pkgconfig` and that the existing output remains useful.

Written by the indexing model from the issue text.